You may want to check whether the missing downloaded songs are just not viewable at the momnt.
You could check: Settings > Music >
If "Show Apple Music" is not on then toggle it on.
If "Show Apple Music" is on, try toggling it off and back on again.
or,
If you haven't already: in the Music app: In the Library:
try checking Playlists, Albums, Songs, TV & Movies, or Music Videos
to see if the missing songs have been sorted into categories.
You can control the sorting of sings: Eg: Play music on iPhone - Apple Support says:
"Sort your music
- Go to the Music app
on your iPhone. - Tap Library, then tap Playlists, Albums, Songs, TV & Movies, or Music Videos.
- Tap
, then choose a sorting method, such as Title, Artist, Date Added, or Release Date."
Else: If you are a Music subscriber: Perhaps Apple Support's online trouble-shooting article may be helpful:
If songs are missing from your music library after you turn on Sync Library - Apple Support
You may be able to check on what your "Recently Played" in the Queue ?
Queue up your music on iPhone - Apple Support