Hidden blade's unseen strike not triggering Mjolner correctly.

So after my latest set of investigations, I can confirm there is a bug, although it's not what a lot of people have claimed it is.

A lot of misinformation has been floating around about Unseen Strike, and several times people have reported things that are correct behaviour to me as being bugs.

The wiki used to claim that it always attacked with the main hand. This is not and has never been correct.
The wiki currently claims that it "locks in" one hand to use each time you gain phasing. That is not and has never been correct (and gaining/losing phasing does not itself have any influence on the hand chosen).
However, it is possible to engineer specific circumstances that would look like either of those behaviours, at least for a while.

The original design intent, for obvious thematic reasons, was that the triggered skill would attack with the Hidden Blade weapon. That weapon is literally the hidden blade that strikes out unexpectedly, represented by the triggered skill. But it has never actually worked that way. Right from the initial implementation, it has alternated hands if dual wielding - not because of any specific implementation in the skill, but because that's default behaviour for all attacks, and the skill has never had any override to that behaviour. While it had not been the original intent, and is thematically dubious, this behaviour was kept - a decision made before the item was ever released. There are comments in the original implementation ticket noting this behaviour and its consequences, both in terms of allowing the Unseen Strike skill to cause on-hit effects specific to the other weapon (such as Mjolnir) and allowing it benefit from weapons with high damage but low attack speed. The unique item, and the skill, were balanced around this behaviour before being released in 3.12.0.

However, alternating does not necessarily mean that each trigger of the skill will use a different hand from the previous trigger of the skill - the alternation occurs over all skills of the character, not just this one. Each action the character does gets a unique action id, which is 1 higher than the previous action's id. Attack actions that don't implement some more complicated handling for dual wielding simply check if their id is even or odd to determine whether that action uses the main or off hand. This means that whether a given trigger of Unseen Strike uses the same hand as the previous trigger, or the other hand, depends on how many actions the character does between the two. This has always been the case since before the item & skill went live, and is still true today.

Unfortunately that is not the end of the story. Technically-minded readers are no-doubt already composing replies pointing out that using action ids this way has a flaw when one attack that should alternate triggers another skill, and indeed this was once the case. Back when Mjolnir was first implemented, if you had 1 or 3 lightning spells socketed for it to trigger, it would stop your attacks from alternating. You make a main-hand attack with Mjolnir, and lets say that attack has id 2. If it triggers 1 spell, that spell has id 3, and then when you go to attack again, that attack action gets id 4 - which is even, making it another main-hand attack. The same principle applies with 3 triggered spells because that's also an odd number of triggers happening. That effect was real and was used by some players to trigger Mjolnir more while benefiting from global stats on an off-hand weapon that didn't get used. Even if moving around and doing other things made one of your attacks get an odd id, that wouldn't trigger anything, so it would alternate back to even on the next attack to restart the "every-attack-uses-Mjolnir" train. This was fixed back in 3.7.0 - a long time before Hidden Blade was even thought of - by having skills that are triggered from another action you perform cause an extra action id to be consumed, so that trigger didn't change whether the next action id would be even or odd. In contrast, triggered skills that were triggered from outside of the skills you were directly performing, such as things triggered by Cast on Stun (or Unseen Blade, but it didn't exist yet), had no need to do this - they were a new, independent action, so used up 1 action id like any other, and fit into the character's general alternation of hands.

This system worked well for a long time, both before and after the release of The Hidden Blade and its Unseen Strike skill. But was complicated in 3.21.0 by the release of Summon Triggerbots, which intercepted an event of triggering a spell to make it trigger twice instead, which of course messed with the number of ids consumed. This was quickly noticed and fixed in 3.21.1 - the patch note specifically mentioned Poet's Pen because that's what it was noticed with, but the bug and fix technically affected all triggered skills. However, that fix had an unintended consequence, which made all triggered skills, not just those being triggered from other skills, now consume an extra action id. This affected Unseen Strike, changing it from the correct behaviour, where it would alternate hands if an even number of actions (including zero) happened between triggers, and the same one otherwise, to the reverse - so if you do no other actions, Unseen Strike no longer alternates.

So Unseen Strike changing hands or using the same hand as the previous trigger is dependent on how many actions the character performs between triggers, as it always has been, but currently it is alternating for an odd number of other actions and keeping the same hand for an even number (including zero), when it should be the reverse. During frantic gameplay, where the character is doing a lot of things very quickly, the number of actions performed between triggers is fairly arbitrary and this is likely to be hard to distinguish from the correct behaviour for most players - especially when moving around, because when one move action transitions into the next - and thus how many action ids are used running around the place - depends on a bunch of internal factors. But in specific circumstances where the character is not doing much or anything else - such as when a player is trying to test this behaviour - it becomes easier to distinguish. A character doing nothing but triggering Unseen Strike will see it always use the same hand until they do another action, at which point it will swap to using the other hand, and this is clearly different from what it should be doing in that case, which is alternating with each trigger.

We are currently internally assessing the ways this bug could be fixed and what else might be impacted by doing so, and potential other improvements to Unseen Strike's handedness.