For some reason (reusing seeds?) both chests in an Abandoned Mine structure have the same loot. Each individual instance of the structure is different, but within a structure the loot is always the same.

Is this intended? Both chests had different loot before.

This may also be a Forge issue, as I think it worked with earlier versions.

MC: 1.12.1
Forge: 14.22.0.2467
RecComplex: 1.4.5

I found that this happens with other structures as well.

Doesn't seem that way for me. Can you test if this happens more often (might want to set the spawn weight of SmallAbandonedMine up to 10000 via /#edit), and if it happens if you spawn it via /#gen?

It only seems to happen sometimes. Some world seeds don't have it happen, while some do. It's weird, to say the least. I'm guessing some random generator somewhere is glitching with some input seed.

I can live with it (I often get lots of golden apples from chests due to this).

Oh, that's a really good theory!

Can you give me the seed of a world where it happens?

I'll have to find one. I'm currently testing a possible fix for a long-standing OpenComputers bug (where computers get corrupted when reloading a world. It seems to have been a mistake of using an array length (1-based) in a for loop (0-based)).

Also, not all instances of a structure have this happen, but most do.

I've recently found out that this is actually a Vanilla bug.