Songs disappeared from playlist

I've had a playlist for awhile that I throw all the songs I like into.

For the past year I've been buying songs from places other than the apple music and putting them into my playlist using my laptop.

A few months ago, most of the songs from my playlist just disappeared leaving only about 20 of the most recently added songs. I tried adding the songs back yesterday, and then added another song from my computer today and the same thing happened.

What would be causing this?