Today I encountered a mineshaft that generated with shulker boxes as part of it's walls and floors. I can't imagine this is a conscious decision since this is a late game item that now you can find in the early game in abundance, so I believe this is a bug.,

I just found the same in my game! Tonnes of shulker boxes, misplaced trapdoors and hanging signs and completely flooded with water... I do have many other mods but not seen any issues with any of the other generated mineshafts. If you would like a modlist or any other info let me know!

Edit to add - fabric 1.21.4!

Edit to add - upon exploration, it seems to be flooded bc it is in a dripstone cave - many of the trapdoors are waterlogged. Part of the structure has generated normally but most is corrupted. Minecart loot chest seems to have generated normally though have only found 2 according to the map which feels low compared to other mineshafts.

Edit to add - so I came across this... Not sure what vein it's supposed to be.. but being in a plains biome, this doesn't seem right either...