I went into Raid Mode, beat up a couple mimics, and decided to test the item-equipping thing. (It had been a while since I got that bug.)
I can equip Golden Thread to the yellow heroes fine. I also got Wine, which I tried to give to one of the red heroes but which went straight to the first gray hero. When I leveled up that gray hero I successfully gave it to one of the orange heroes, then tried a red one again and it went to the gray hero.
Weird edge case, but still.