Tried a few things so far, doesn't seem to be vanilla or modded race, 1 of modded race works, the other doesn't even though they're identical in genes, gear, implants, etc, literally only difference is marine armor vs prestige marine armor, swapping shields between them doesn't matter, the other stays broken, same for armor swapping, no effect.
Tried messing with order of equipping gear, other races, but I can't find a stable way to reproduce or notice symptoms.
Modded armor also doesn't seem to matter.
Maybe it was cause of pawns that went into gene extractor, but no, doesn't affect others that went into it.

It gets even weirder, I tried removing a few mods that I added in the last 2-3 months, but that made no impact.

Now I was testing about layering, for 1 pawn the shield correctly showed up infront instead of behind once she was stripped naked.
So I also tried this one the other pawn, but even when that one is completely stripped the shield still appears behind him.
I've tried 3 different shields, buckler, assault and carapace, it's not the item cause it happens with all 3 and all 3 work fine on other pawns.

I might've found it, the issue is ocurring on pawns that I had before I removed the "melee animations" mod in this playthrough, they also both had a lasso equipped.
I'm adding it back now and will give them a lasso again.

Nope that also wasn't it, with the mod installed, with or without lasso doesn't change anything.

Okay it gets even weirder.

So I noticed for both of them the shield is only fully visible when viewing their left side (they moving left).
So I checked a pawn I thought was working well, it shows the shield when moving left, right and down (but obscured when viewing the back as they move up), but thus would mean the shield moves to the different hand.
So I checked another pawn, on that one I only see the shield when they're moving left and down (which makes sense since the shield is on the left arm and I think this is expected behaviour)

At this point I'm starting to go nuts lol.