Bundles hold four spaces in another bundle. Not sure if this is a bug it just seems weird because bundles are 1 inventory space
MC-207234
Bundle's hold 4 spaces in a bundle
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Hey there!
This feature works as intended. Bundles can only hold a stack's worth of items as quoted by [Mojang] Adrian Östergård in this link. Bundles can stack up to 16, therefore making them take 4 spaces in a bundle. Just to get you familiar with how this item works, I've quoted some things mentioned in this link above. I hope this helps clear any confusion.
A bundle can hold a mixture of items, but only one stack’s worth. For example, a bundle could fit:
64 dirt
32 dirt + 32 stone
32 dirt + 8 ender pearls
One of every color of wool, concrete, carpet and terracotta (64 blocks in total)
Or one diamond helmet