Until at least v3.05, Mp3tag showed a warning when editing a read-only file.
Merely renaming a file was done without warning.
I believe that was the intended behaviour.
FWIW, Windows File Explorer does not allow metadata to be edited for read-only files.
But v3.05b, and later versions, do not warn when editing a read-only file.
Instead, the read-only attribute is removed and the metadata is up-dated without warning.
I can't see anything in the change logs to indicate that is a new intended behaviour.
If it is not a bug, I think the pre v3.05 behaviour should be reinstated.
There should be a warning when trying to edit a read-only file.
I suppose the warning could be optional.