So it turns out that even in recent versions, all music played using the /playsound command has a chance to randomly stop about halfway through. The main cause is unknown; however, one cause is when a different sound, or more than one sound is playing. More context is shown in the original description, which is shown below. I thought that the bug only happened in the betas, but it turns out that while playing on my Switch, the manually played music cuts out out of nowhere as well.

Original Description:

The new nether songs are amazing. However, when using the command below to play Rubedo, it sometimes stops playing in the middle of the song. It does not happen every time, but happens around 50% of the time.

Use:
/playsound music.game.nether_wastes @s ~ ~ ~ 100

Edit: Apparently while using the same command (except instead of playing Rubedo, I played Chrysopoeia), I looked at an Enderman in creative mode and the music stopped playing. This may confirm that other sounds stop playing the music; possibly more than one playing at the same time, because there already is a known bug with the Enderman that makes the screaming sound multiple times when a player looks at it in creative mode MCPE-42977 .