Editor discards changed code when closing tab

All,

This relates to Toad for Oracle 12.12.

From a windows directory list I dragged a few hundred files into the editor. Works fine.
I changed them one by one, close the edit tab, get a question pop-up: save Yes/No ?

Which I answer with Yes.

On one of the files, the editor ‘forgets’ to issue the pop-up, hence the changed source code is not saved.

Did I double-check? yes: all files had a change-date within the last hour. Except one, which had remained unchanged.
Re-applying the change then exiting works okay: I do get the pop-up and the file is saved.

This happened twice this morning.

Would you please have a look at this issue?

Thanks in advance & kind regards,
Abe Kornelis