What happens if you restart MC or reload the library? Are the same files missing? Were they definitely ADDED to the library? It is possible to view and tag files in MC but not actually add them to the library.
If you believe the records are truly in the library, not just listed by MC, then are they firmly in the library view you are using? If the tagging changes a tag that is being used by the view to select records, the changed record will then become unselected and disappear.
More thoughts, based on experience + speculation about MC's workings...
Changed tags therefore view deselection can happen even if you do not change a critical tag, because a tags update from library back to file can overwrite most or all of the tags, not just whatever you manually changed, and some tag changes are based on defaults or expressions. For instance, if there's a file naming/renaming formula somewhere, it can get applied to effectively move the file. If the file isn't actually in the library, MC might not keep track of the new location.
There seems to be a small view oddity, where a view "loses" some records, but the records are still in the database, they just aren't listed in the view. Usually F5 to refresh the view gets them to reappear. This behavior seems to happen when the view is sorted on a field/tag and that field is then edited, or a record is deleted. OTHER records with the same sorted value can disappear from the view. This mainly happens to me in a view sorted by song title (Name field). I'm reviewing possible dupes or otherwise working in a view where the same song title is listed multiple times. If I delete the currently-playing song, sometimes ALL the other records with the same Name vanish from the view, but reappear if I press F5. Or sometimes SOME of the other records with the same Name disappear. This also happens (but less often) if I delete another record with the same Name as the currently-playing track. I believe this is related to MC's need to update the index of the sorted field because I've changed or removed a value, which apparently doesn't always update unless I reload/refresh the view. It's scary to see records vanish but after a while I've become comfortable that it is just cosmetic.
Also, possibly the same behavior, I've encountered views where records come and go even before I make any changes. Yesterday I worked through a large set of records in a view subset (such as 400 records in a view sorted on custom field Artists and sub-setted as Artists > B > Beatles), reviewing, tagging, deleting. Then I moved to another view not at all related to the records I was working on. Then I returned to the original view subset, and found it had a few records I somehow "missed" the first time through. I'm convinced I didn't miss them -- they weren't in the view the first time. This behavior happens periodically, and is probably another instance of the view index not being updated promptly, so I press F5 a lot to be sure of what I'm seeing.