The problem keeps happening, and I've discovered odd MC 12.0.520 behavior that, if accurate, is scary:
I wanted to update some tags in a song that has long-existed in my library, and I had recently played without problem.
Tags I changed: Artist, Artists (custom field), Genre, Keywords. Also, I removed cover art that was incorrect.
Suddenly, while saving these changes, MC reported that the file was missing! (MC is set to store tags in files.)
I noticed the database record's filename had been CHANGED to a new path. HOWEVER, I did NOT invoke Rename/Move, and the file was NOT actually renamed/moved. Somehow, MC decided to change the database record's file path, but without actually changing the file, it immediately got "lost"!
There's logic to the new path. It reflects where the file might have been moved IF I had invoked Rename/Move HALFWAY through tagging, because my file path is an expression that uses some of the tags I changed. But the "new" path is only partially correct. It includes the new value of one of the tags I changed, but only part of the value of another tag I changed. I believe this indicates that after I started to change these fields, the file path got mysteriously updated mid-change, so my subsequent changes triggered the "can't find file" error. At no point during tagging did I click Rename/Move. In fact, I never invoked it for this track because the error popped up first.
Of course, once a file gets disconnected from the database record, nothing more can be done because tags can't be saved to the file and file properties can't be restored to the tag. They no longer are connected. I repaired the record by locating the physical file -- still at the original location -- then copying the real location back into the database record. But why did the database's file/path get changed yet the file itself wasn't?
Key question: Did I inadvertenly click Rename/Move? I don't think so. Two proofs: I was working only in the Tags action window, and didn't touch any MC library actions except for clicking Remove Cover Art -- one click, and it got removed. IF I had somehow also clicked Rename/Move, why didn't the new file location reflect my expression for naming it? Instead, the changed path reflected only partial tag changes, as if Rename/Move was clicked mid-way through changing the tag values, which it wasn't (unless there's a mysterious hot-key I don't know about). And, since this problem has happened MANY times recently (about 1 out of 30 tracks tagged while ripping about 100 CDs), and almost none of them had any "wrong" cover art to remove, I didn't go anywhere near Library actions until well after all tagging was completed. (My normal practice is to rip to a temp location, then tag/fix everything, then after careful review, click Rename/Move as the final step.)
More oddity: Every time MC told me this file could not be found, it also started listing other "lost" files -- one dialog after another, as I click "No". The files were those that became "not found" yesterday when I ripped 300 tracks and had this problem about 10 times. But I fixed those, so why was MC still complaining, and why do it while reporting a problem with a particular track, the only track I'm tagging? I'm NOT clicking on any of these "lost" files. But when I click on ANOTHER "lost" file, I get the error message about one of yesterday's "lost" tracks. What's with that??
By the way, "normal" behavior when I click a track that is disconnected from the database is that MC plays another random track instead. This is odd, and makes me think the track is misnamed until I realize what's happening. It takes some time to determine what is actually playing vs. the clicked track.
One thing to note: In my Rename/Move expression, one data item is a multi-value field. MC converts the semi-colons to underscores, but otherwise seems happy to use this field. But could the nature of this field trigger the problem? Just in case, I changed the Rename expression to use a single-value field.
Can I get this mysterious behavior to repeat via experiments? Not so far. I change tags and add/remove cover art, and watch the file path and it doesn't update unless I click Rename/Move. But it definitely did change, many times, so I can only report this as a mystery....