I found this behaviour when trying to edit the M4A file found here.
It seems that Mp3tag creates a temporary file of an M4A file being edited.
Maybe that happens all the time, for all file types.
When Mp3tag cannot open a file for editing, an error message is displayed that asks if I want to retry.
After clicking "No", the program returns to the "Ready" state.
However, a tempory file (eg. TMPB239.tmp) remains, beside the file being edited.
For me, that is undesired behaviour.
I expected the temporary file to be deleted.
Maybe it is a bug.
Windows 10 Home (64 bit), Mp3tag v3.18f (32 bit)