You should really report that as a bug in the MC21 release thread: http://yabb.jriver.com/interact/index.php?topic=99729.0
I'm pretty sure when a MC Client is opened, the whole Server library is copied to the Client.
Yes, I think so ... in a way thats sort of a temporary "virtual" file right? this is why, if you shut down auto-sync and you forget to manually sync, your changes will be over-written on re-connect.
However, once a Client is connected to the Server, synchronisation should happen at the record level, and any record that has been modified on the Client should be marked as protected from overwriting until it has been synchronised to the Server. I'm not sure the MC's Client Server implementation does that, but it should. Maybe something has been broken at some time.
I'd like confirmation of this if true on the client side ... its definitely is on the server side... I think it more or less works that way for a client, but it is all the "records" that one has selected, one track, one albums worth or even one album artists worth of records. It is my understanding that this virtual library file that is loaded, allows for a lot of speed for really big collections and reduced network access, as well the ability to protect the server library when multiple clients are syncing too it (ie. updating playback stats on one client and someone retagging on another.)
Once the tag window info is confirmed, and/or something else is selected, I'm hoping you are right - it appears to be protected, (euh, most of the time, but I don't trust it
)this could take more than two minutes, sometimes a lot more. . If you happening to be listening or zapping through what you are currently tagging and the track changes, this also could be a trigger.
Some examples of where I can reproduce this overwriting ... I select all tracks on a 5-CD opera ... I then use my link bar go to various websites, add the orchestra add the "work" name, original recording date, and the soloists and if they are singing mezzo-soprano, and the roles they are singing .. Another example is when I add a new album of a group I already have music, I use those "magic arrows to flip to another view that will show them all, I might use pane tagging to change all or just make the new addition the same .... all of these changes take more time than usual, and can be overwritten in my experience.
On the server side these records are always protected, but the reason for tagging from a client is that you are using your TV or are running headless, so little/no admin is done on on its library directly. If you happening to be listening or zapping through what you are currently tagging and the track changes, this also could be a trigger. It could happen periodically after a certain period of time, if nothing else happens too, not sure. This could explain why one little thing that you just changed gets over-written by the server data. There are a bunch of posts on this if you dig, but I've never read any confirmation on how this actually works.
I'm running Media server with DLNA options running, and I often listen to what I'm tagging at the same time, for info. This is as I said not new, I have been using this manual sync at least since version 16 to avoid this. I just don't think its broken ... maybe its a compromise that doesn't affect most people? I'd be very interested to know what triggers the sync and how its set-up to work via the clients