I have already checked through the existing feature requests and found no duplicates

• Yes

Is this a server-specific feature?

Jellyfin

What do you want to be added?

After #1439 a lot of song's album cover cannot be shown up due to missing of Primary Image

http://<server_url>/Items/<item_id>/Images/Primary?quality=96&width=300

Will give 404 as expected like this.

Failed to load resource: the server responded with a status of 404 (Not Found) "<item> does not have an image of type Primary"

However, an album image is present and the software will happily use it before #1439 was implemented.
When you have a large database you'll need a lot of time fixing this issue. I have no idea if it's Jellyfin's fault or something, it should use album image if no Primary is specificed for song.