That honestly sounds more like an Apple Music sync/library bug on iOS 26.5.
The fact that older songs save normally but newer songs disappear is the weird part. Usually that points to a cloud library syncing/indexing issue.
A few things to try:
- Go to Settings → Music and toggle Sync Library off, restart the phone, then turn it back on
- Make sure the songs are actually added to your library before adding them to playlists
- Try adding the same songs from another device or from desktop Apple Music and see if they stick
- Check if the songs disappear only on your phone or everywhere
- Sign out of Media & Purchases, restart, then sign back in
Also:
- Some newly released songs get weird regional/licensing sync delays for a while after release
- If the playlist updates instantly disappear with no “Added to Playlist” confirmation banner, that’s usually sync failing in the background
Since it started recently and only affects newer tracks, I’d lean toward a server-side/cloud library issue rather than corrupted local data.