PoE2 - Mercenary skill vanished at level 41

I upgraded my flash grenade to level 10 in the middle of a dungeon. When I hit the hotkey I noticed it wasn't binded anymore. When trying to rebind I noticed the skill wasn't available for selection.

When I hit 'G' I saw the skill was still slotted, but when hitting the arrow to show the stats it doesn't show the stats for Flash grenade but for a different skill.

I relogged and closed the game and started it again but it doesn't change anything.