I was having the same problem consistently since I started using mp3tag. I think I may have just stumbled on a solution, so far it hasnt happened today once while editing 1000's of files. It may have to do with the shmedia.dll windows component that constantly opens and reads metadata from media files in which the system is currently involved. Its functions are almost always laggy and delayed while keep the file locked for a ridiculous length of time.
I used nirsoft's RegDllView utility to temporarily backup the registry entries for shmedia.dll while using mp3tag, then restoring them afterwards.
I also disabled foobar's monitoring of library folders, and manually update the library after doing tag edits. This may also help the locked/tmp file issue. But I think more importantly this fixed a problem where almost every edited file was readded to the library, overwriting the Date Added in playback statistics component.
Hope this helps