Drowned mob dropped a chest upon death

I was swimming in an ocean in my world, when I encountered a drowned that had nothing in its hand. After I killed the drowned, it dropped a chest upon death. I opened the chest and it had 6 other chests and a piece of paper inside of it. before I placed it down, it had a shulker box-like pop-up that showed its contents. I didn’t take a screenshot of what it looked like before I opened it but it looks something like this attachment.

EDIT: It happened again with another drowned, I’m not sure how. This hasn't been a bug for me before. Again, I don’t have any mods or data packs installed, just a resource pack, which shouldn’t change drop rates.

EDIT2: About the second chest, 5 of the chests were empty, but one of them had a fishing rod inside of it. The more I look into this it just gets weirder. The 2 chests that had items in them stack together, and the empty chests stack together. But the chests that had items, and the chests that didn’t, don’t stack together to make 7 chests, they just act like two different items in my inventory.