Signature in mail does not work properly in MacOS Ventura 31.1

Hi there pmj49,

If you haven't already, you can see if quitting the Mail app (Mail in menu bar > Quit Mail) and restarting your MacBook Air may help. Doing that will refresh both the Mail app and macOS, which may help resolve this issue. If needed, below shows how to restart:

"Restart your Mac

• On your Mac, choose Apple menu  > Restart.
• If you don’t want app windows that are open to reopen when your Mac restarts, deselect “Reopen windows when logging back in."

Those steps are from this article: Shut down or restart your Mac - Apple Support

Another step you can try, is to see if this happens in safe mode as well. Safe mode only allows certain software to load, which can help isolate this further or possibly resolve the issue. If needed, the steps below show how to get into safe mode:

"How to use safe mode

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.

Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window."

Those steps are from this article: How to use safe mode on your Mac - Apple Support Please let us know if those steps helped or if you've already tried those steps.

Take care.