I made the mistake of clicking on a piece of gear already on my character. I could not reslot it.
I exited to the character select screen and came back, but the gear was still in limbo between being equipped and being placed in my bags.
After placing in my bags, I was unable to re-equip.
I assume this is not a bug, but rather some kind of change to the game that was intentional.
I don’t understand it though, and it is not explained in-game.
A quick search seems to indicate there is no way to auto-merge the tabs and gear from the different dropdowns into Legacy, and I must drag and drop from “Remove Only” drop downs into my bags, and then can place them from my bags into “Legacy”.