Inaccessible Items After Map Completion

After doing an Enslaver map that had a delirium mirror I was faced with a huge problem. During the 2nd of 3 transitions/phases of the final boss fight for the map, the mirror popped. All of the loot I was going to get dropped on the ground. I had my mines already on the ground and they all annihiliated the boss into the next phase, which pushed me into the 3rd room/phase. Now I can't go back to get my loot. Shouldn't there be a way to travel to the previous 2 other platforms/phases/rooms for me to recover what was dropped? This has happened more than once.

I assume from the lack of replies this is somehow working "as intended"?

Sadly, yes. Has worked this way for years. The only workaround is to force end Delirium before you enter boss arena so it spawns in the fixed zone instead of the boss transition zones.