Nether music does not play in Nether biomes in customized worlds; instead, overworld music does

The bug

I loaded a Buffet world in Floating Islands Basalt Delta and as I was exploring it to fill out a map, I noticed that I was not hearing any nether music, be that any of the old 4 or the new So Below. Instead, I was hearing Overworld music, and I don't think this is intentional, especially since other new nether sounds were played.

How to reproduce

1. Load up a buffet world with any of the new biomes. I have tested Basalt Delta, Soul Sand Valley and Warped Forest in Floating Islands, Caves and Single Biome, but I figure it is likely that it's similar in other combinations too.
2. Wait around in the Buffet world and Overworld music will play; even if you wait longer, only Overworld music will be heard.