Edit HTML signature in Mail 16.0 on MacOS Monterey 12.4

Since a few day, my HTML signature are gone on _Mail_. I used to edit manually the *.mailsignature files located in the ~/Library/Mail/V9/MailData/Signatures/ folder and locking them to prevent _Mail_ to override them. Unfortunately, since the last update, I can't select it with the combo box: Note that I can't select a signature created manually either.