When a bundle inside of a bundle (or a shulkerbox) is burnt, items inside drop on the ground duplicated.

this can be seen easily when each bundle (inside of a bundle/shulkerbox) is filled with a netherite ingot (one of the items that don't burn and can be picked up after the duplication)

duplication seems to be randomized, but with a high success rate

1. Get 3 ingots and 4 bundles in creative
2. Put 3 ingots into 3 different bundles
3. Fill an empty bundle with netherite ingot bundles
4. Throw the bundle into a fire (this might have to do with multiple fire next to each other)
5. Pick up ingots from the fire (If picked up 3 ingots as expected, repeat the process. Duplication will happen eventually)

Video

https://bugs.mojang.com/secure/attachment/359566/awesome%20game%20design%20right%20there.mov