When you use a resource pack with custom music disc names in the lang file (item.minecraft.music_disc_name.desc), these music names are not shown in game. Instead it uses the vanilla providden music names.

In the example below, we have the pigstep music disc. In 24w20a as other versions, it shows up as "NCS - Make Me Move (pigstep)".

But in 24w21b, with the same resource pack, it shows up as "Lena Raine - Pigstep" instead of  "NCS - Make Me Move (pigstep)" as it should be.

Update: Haven't noticed that music names location have been changed, Please mark this bug as invalid