Play a music disc, then either change the resource packs you have loaded or press F3+T to reload the resource packs. The music disc will stop playing and you have to eject and place it again in order to hear it.
Environment
Windows 10 x64 Minecraft versions 1.15 and 20w17a
Linked issues
is duplicated by 12
MC-192344
Jukebox stopped playing
Resolved
MC-200121
Minecraft music stops for a period of time after reloading
Resolved
MC-254548
Music stops upon performing an intensive task
Resolved
MC-264407
Jukebox Stops Playing when Reloading Resource Packs
Resolved
MC-264523
Summoning a warden and then loading a resource pack cancels out the digging sound
Resolved
7 more links
This is intentional, the whole sound engine gets restarted when you load a resource pack, or do F3+T, furthermore, the resource pack may change the music.