I have been annoyed by the same issue (and I too believe it was introduced in Patch 4).
Characters with less than 10-Strength have difficulties with some medium/heavy armours, they cannot : pick-up them up, carry them in inventory, and equip them.
It doesn't affect all medium armours. Shadowheart's armour goes on all 8-Strength characters. Some heavy barrels like the Mind-Flayers' oil barrels are concerned as well (can't pick up, can't carry). I'd wager it's based on the weight of the concerned items, though I have not bothered finding the threshold.
As Maximuuus said, this is a "custom rule" : in 5E, only heavy armour has strength requirement. Or maybe it's a bug. With Larian's minimal communication, and Patch/Hotfix notes that are far from exhaustive, it's hard to tell whether something is Working As Intended or is a bug (so long for quality feedback).
Should it be WAI, I see the negatives it brings.
- It makes no sense, immersion-wise : a character with 8 Strength can carry a total weight greater than that of those items they cannot pick up.
- It's a strict decrease in Quality-Of-Life.
I fail to see the positives. I am in hope that it isn't a hasty measure to limit barrel cheese (making barrels targets was bad enough), or prevent mix-maxing (which Larian usually wouldn't bother doing).
All I can say is that Larian works in mysterious ways ...