I have enabled the 'signature tweaks' lab. Nothing more. No automatic signature text. I wrote an email to myself and added my name manually. It appeared greyed out under my text. That's fine.
Then I wrote another email, with my name and something extra written underneath, and my name plus the added text appears, not trimmed off and not greyed out.
Looks like enabling the 'signature tweaks' lab resolves the issue, but I agree, it shouldn't have happened in the first place.