The new music wasn't updated for the fallout mash-up pack

How to reproduce:

Go underwater in a water biome

Go to the nether and its biomes

Go to the new cave and mountain

Wait for the music added in 1.16.0 (nether biome specific music, the 3 underwater themes) and the 1.18.0 music (new mountain and cave biomes) to play, and you'll hear that they're exactly the same as default.

This is incredibly jarring as all the other music tracks (excluding the music discs) are all fallout themed.