I've made a small resource pack with custom music for the music discs when 1.16 were released, which included edits to the names of the discs and their descriptions when using either English US or English UK. At some point (might've been as early as 1.20, but I can't confirm that) the custom names and descriptions stopped working. This holds true in both 1.21 and 1.21.3, and presumably all other sub-versions of 1.21 as well.

I've confirmed that the .json files have correct syntax, and that they are pointing to the correct entries in the .lang file. When doing this, it came to my attention that you can now edit the jukebox's on-screen announcement showing what song is playing separately, and when I tried editing those entries it also failed. The only thing working is the music itself, so the problem seems to be localized purely to editing the language entries of the discs. I can also confirm that editing other item names in the .lang files still works like normal.