Why are my songs disappearing from Apple Music?

I created a Christmas playlist with hundreds of songs to share with my aunt (we both subscribe to Apple One), but shortly after I completed the playlist, I checked on it and found that the majority of the songs had disappeared without explanation.

I thought it maybe had to do with some of the songs not being Apple Music file types, so I went through the painstaking process of finding all of them on Apple Music and adding those to a playlist (instead of adding my own existing songs, with their various file formats).

This playlist ended up containing 437 songs. I shared it with my aunt as a collaborator so she can add or remove songs as well. She hasn't added or removed anything,and on her phone and mine, the playlist still has 437 songs. But when I go to music.apple.com, 300 of them mysteriously disappeared, leaving 137 songs. Why does Apple Music stink so much compared to the perfectly awesome iTunes? This is a truly awful experience--and it's one we're actually paying money for! Why is this happening? Thanks, all.

[Re-Titled by Moderator]