Add photos (DNG and JPG) to album and let them syncrhonize with Adobe cloud
Wait (a month?) to make a fresh edit (doesn't matter if photo was previously edited or not)
Photo instantly disappears from album (and all photos in Lightroom Mobile) as soon as you back out of the develop mode
Expected result: Photo stays in Lightroom Mobile showing latest changes Actual result: Photo instantly vanishes without a trace on Lightroom Mobile, but there IS a workaround. These photos had already synchronized with Lightroom Classic on my PC. Since I have had Lightroom Classic open while I have been editing the photos on my phone, they almost instantly appear in the "Smart Collections" "Recently Modified" folder. And the latest edit I made on my phone is present in those changes.
What's interesting is that it disappears from the album on both Lightroom Mobile on Android and Lightroom Classic on PC...so, what I do to get it back, is move it back from "Recently Modified" into the album on Lightroom Classic and let it sync to the cloud. Once it does, it's not long before it reappears in the album on Lightroom Mobile.
One potentially related issue to this (or perhaps may suggest a possible cause for this bug) is that I initially uploaded these photos while I was in a different time zone (in this case, when I was in Tanzania). The capture date is relative to the time zone and the filename indicates the time from when I was there. When I fix SOME of these photos using the "Recently Modified" workaround in Lightroom Classic, these photos still reflect the proper capture date. However, I've also seen the capture date change on some to reflect my current time zone (so the capture date no longer matches the filename).
Note that I have not seen this bug occur with any of the 7000+ CR3 photos I uploaded to image.canon and synchronized with Lightroom during my trip to Africa. It has only affected photos I took with my Galaxy S21.
I can now confirm at least one (possibly more) photos that I had added from my Android phone to Lightroom Mobile have completely disappeared after making edits. There was one I thought I had exported and planned to share today, but it was nowhere to be found in the AdobeLightroom folder on my phone. The original no longer exists in Lightroom Mobile's library, it is not synchronized with my PC's Lightroom Classic, can't find it in the web version of Lightroom. The original dng (which I had added to Lightroom) and the jpg that got concurrently created when I snapped the photo with my phone both do not appear within DCIM\Camera folder on my phone (where all of the other originals exist), nor do they appear in the Gallery. I cannot find the file in the trash. It's like it never existed. I have a ballpark idea of the filename (which is based on the capture date). It does NOT come up in searches. It's like the file never existed!
And since I made those edits when my computer was not on, I don't see it apper within the "Most Recent" smart collection (which was my way of fixing the previous photos I mentioned that stopped appearing in synchronized albums).
How can this be? Is there ANY way to search the history of syncrhonized folders within a Lightroom album on Adobe Creative Cloud to find files that may have been in the cloud but are no longer associated with any Lightroom album?
Ok...it turns out that this particular photo was from my mirrorless camera...which I may have uploaded from my phone (because I did copy some from my camera to my phone for more immediate work) or could have trasferred directly from image.canon (where I initially upload photos from my camera when connected to WiFi). The latter automatically sends the RAWs to Adobe Lightroom over the cloud (which is a great convenience).
This is actually the first instance of one of my mirrorless camera's CR3 photos disappeared from Lightroom Mobile's "All Photos" after an edit. Up until now, it had only witnessed the DNG and JPG that were added to an album disappearing from the "Album" and "All Photos" sections of the app. As mentioned in a previous comment, I found it in the "no keywords" smart collection of Lightroom Classic on my PC. AFAIK, there is no way to see smart collections from the mobile app...
So, as far as the original CR3 file as copied to the phone the goes, in this particular case it's not like it actually vanished from the phone after all. I was just looking for the wrong kind of file of photo. And it isn't as bad as the phone photos since I still have the originals on their memory card (they only stay on image.canon for 30 days).