Since Sonoma my modified email signature are overwritten by apple mail

I was used to edit mail signature files in mac os but since Sonoma apple mail keeps overwriting the files althought i locked them. I changed the file on disked with bb-edit after having closed mail en unlocked the file using info-window. Locked the file again after modifying. Checking file afterwards everything okay. But after opening Apple Mail the previous version shows up and the mailsignature-file gets 'updated'. So, how can I edit signature files outside of Mail?