Okay so I tracked down the "bug" I've been talking about. (lots of testing, phew!) turns out you can't move items from one inventory to another if the recipient doesn't have enough strength to pick up the item in the first place. I'm not talking about carry capacity, I mean certain heavy items are too heavy to be picked up at all! but there should probably be a UI message that tells you why you can't move an item between characters, pick it up off the ground or out of a chest. because I just spent the better part of an hour testing things before I worked out what the problem was.
So yeah, long story short I have been punished for my Min/Maxing, fairly annoying as medium armor isn't supposed to have STR requirements in the tabletop game, but that's what we have.
Edit: cool work around though, you can drink a STR potion equip the armor you need and the armor stays on after the potion effect is over.
Had the exact same experience; I spent an hour trying to work out why Gimblebock's armor kept vanishing when I looted it; reloaded and reproduced the behaviour twice, then on the third time decided to loot items individually and saw it stay on the corpse. There really does need to be a UI prompt for that kind of thing.