Hello lozuk,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with the Mail app removing your signature from composed emails. First, try restarting your Mac to see if that resolves the issue: Shut down or restart your Mac - Apple Support
"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.”
If that doesn't help, does this occur with more than one signature, or in more than one account?
We hope this helps.
Cheers!