I've encountered this a number of times. It usually seems to happen when MP3TS tries to write the tag when Explorer is showing its ``preview'' (which also often prevents one from renaming the file within Explorer itself -- POS!).

Anyway, when this happens, the only way I've found to fix it is to exit MP3TS, copy the file, delete the original, make sure that Explorer isn't previewing anything, and then try again. Sometimes I still have to tell it to delete the entire tag first before it'll work right.

I don't know what MP3TS's problem is, but even after making sure that nothing, including Explorer, has a handle on the file, and after closing and restarting MP3TS, it still fails to be able to change the file. But it can change that exact copy. It must be caching something somewhere.

Edit: Oh, and it's not specific to the new MP3TS.