Album art displays erratically in Android app

Display of album artwork in the Android app for my local music library is very erratic. The library is stored on my Synology NAS. I have 100% of my album artwork embedded in the files, in sufficiently small-sized jpg. Also, I have a folder.jpg in almost every album folder. None of the jpg artwork is progressive.

I've done a lot of experimentation with this subject, and have found:

1. Album art sometimes shows up in the app, but often doesn't. (In the Windows desktop app, all artwork shows up all the time, as it does in Windows Explorer.)
2. Some albums are often displayed with their artwork, others are often displayed without, but almost all albums are shown with artwork at least some of the time.
3. There is no relation that I can detect between the size of the folder.jpg, or the size of the embedded artwork, and the display of the artwork in the Android app, nor with the number of tracks.
4. Albums at the top of the alphabetical album list in the app are most often shown without their artwork. Almost as if the app needs a reason (scrolling into view, say) to refresh the image.
5. Network latency may be a factor: after I set my wifi router to prioritize both my Sonos devices and my phone, all artwork suddenly showed up quickly first time I checked. Pretty soon, though, the old erratic behavior returned. And I've at least 70-100Mpbs Wifi for the Sonos One and my phone, and nearer 150Mbps for the Beam, and the NAS is hard-wired to the router.
6. It may matter how the artwork is embedded. I've had occasions where changing the embedded artwork type to Front Cover instantly solved the missing artwork in the app. (But it's not a certain solution.)
7. The app seems to respond to changes in the files. When I "touch” (edit) the files in an album, it often makes the artwork appear in the app, but not always.
8. The display of artwork changes often. This morning, 150 of my 600 albums had artwork missing in the app. This afternoon, of the top 95 albums in the list, 24 additional ones had artwork showing, while 14 had lost their artwork since this morning.
9. Refreshing the music library tends to change which albums have artwork showing, but doesn't fix the issue.

So to conclude: the artwork seems to show up almost randomly in the Android app, and I've found no surefire way to make it show up all the time.

What's going on here? And why doesn't the Android app simply cache the artwork it's already displayed once? After all, we have an Update Library command in case of changes.

Diagnostics submitted: 89729938.