I have been experiencing the same problem. I have a playlist of songs for my kids, and it seems that songs keep disappearing from it in bulk. I suspect it could be a sync issue, despite the fact that I have the "manually manage" box checked when I plug into computer. I will try to pay more attention to when songs disappear, but one example I can give happened this morning.
Just this morning I added two new albums to an existing playlist with a few albums so that there were over 50 songs. After plugging my phone into my computer and backing it up, one of the albums I just added was gone from the playlist, and all but 2 songs from the other just added album were gone, as were all but 2 songs from the previous albums in the playlist (50 songs reduced to 4). All songs were purchased from iTunes in the last several months, and when I look at the songs in the library they all have the little downloaded arrow next to them, not the cloud icon, so they should still be on my phone. As stated above I have the "manually manage" box checked for music when I backup my phone.
I am using iOS15.6 however I noticed this issue before the last update. This is very annoying, especially when you don't want all songs from an album in a playlist, so repopulating a playlist becomes time consuming, as well as realizing a previously downloaded song is no longer available when you're away from home and don't want to use cell to download again.
Thanks.