When bamboo generates in any non bamboo jungle biome, it will generate regardless of whether it has sky access. This glitch can also happen in any buffet world containing a non bamboo jungle.
To reproduce, use the seed 2612914 and use the following command (if you don't, the bamboo may not appear):
/tp @p -50 24 -1132 -95.5 -3.2Linked issues
is duplicated by 2
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I got information to easily reproduce this from MC-153724, and I hope @Ryan jones doesn't mind that I stole some information from his report.
The caves preset generates differently depending on the biome for some weird reason, but this can still be confirmed if you look around in both biomes for a few moments to confirm that underground bamboo does not spawn in bamboo jungle or its hills variant.