The bug

Cave vines cannot be placed on the underside of leaves.

This makes sense for blocks such as bells, however leaves can support normal vines, so it'd be expected they they'd also support cave vines (and possibly other blocks such as weeping vines) by extension. This, however, does not happen.

How to reproduce

Right click with vines on the underside of a leaves block, and do the same with glow berries.

Expected behaviour

Both of these place a type of vines, so it'd be expected both would be supported by leaves.

Actual behaviour

Normal vines are placed just fine, but cave vines completely refuse to be placed.

Suggested fix

It may be worth adding a tag of some sort, perhaps #minecraft:vine_support_override, which itself contains #minecraft:leaves. This tag would allow any block in it to support all types of vines (vines, weeping, twisting, cave), regardless of any other factors like its solidity or support shape.