So like others, I had the issue that since the 26.4 update, random albums were not displaying their art. I tried various suggestions listed on here, restarting just the Music app on my phone, force restart the whole phone, resynching via iTunes and none worked. I have over 12,000 songs on my iPhone, some downloaded directly from Apple, some from other places, some ripped from CDs, and from various other places, and the missing artwork seemed randomly among all of them. I manually manage my iTunes and what goes on my phone, it's not simply the entire collection, so with the phone plugged in, I click on the device button, then music, and almost always add/remove using the albums option, something I've done going back to the early iPhones which had much more limited memory so I had to do this regularly back in the day.
Anyway, I tried a variation of what IEFNH8Zombies suggested, and eventually, after 3 hours of working around and trial and error, I finally fixed it. I had to do things a bit differently, so in case this may help others, here's what I did to the best of my recollection:
- On my phone in Music, I went through the "Albums" list to see which ones were missing art. You can do this other ways, but as this lists everything, I felt it was the most direct. I made a note with a list of everything missing art.
- Plugged the phone in, and after it sync'd and backed up and everything like usual.
- From the device/music/albums section, I manually unchecked every album that was missing art to remove it from my phone, sync'd, then went back and checked them all again to add them back. There were about 50 albums that were missing art on my iPhone (I didn't do an exact count), all of which have it on iTunes (In some cases, have had it for over 15 years!), and this restored more than half, leaving 18 that still were not showing it on my phone.
For these remaining 18 albums that were still not displaying their art on my phone, after trial and error, here's what I had to do:
- As before, plug your phone in, sync, remove every album that is missing art, and sync again. Make sure to leave the phone plugged in, as you'll be syncing more.
- Go back to the main iTunes music page where you can edit the albums that are missing art.
- For each album, first click on the album to display the track listing (Important!), go to the Album Info and then go to the artwork. Copy the art, then delete it from the album (After copying, I'd just press backspace). The album art should now be grayed out in the full listing of albums, but may still be displaying next to the tracking listings, as shown below:

- If this is happening, go back to the device tab, sync, then go back to the main music listing for the album, and now both should be grayed out. Paste the art back in the album info.
- Go to the next album and repeat the process of copying the art, deleting it from the album, syncing, and pasting the art back in.
- Once the art for all albums has been repasted, go back to the device tab and add all the albums back to the phone, apply and sync changes. This should restore all the art on the phone in all views.
I tried the previously suggested option of pasting the art on the desktop, but as I was essentially syncing after every album, it felt easier to just do this.
As mentioned, simply removing the albums, syncing, then adding them back and a second sync did restore more than half. If you still have albums that this doesn't fix, try the remove art/sync sequence described above before adding them back to your phone. For those 18 albums, it took me about 30~ minutes to complete that process.
Hope this helps. It's insane that this update can somehow break album art out of nowhere...