If you haven’t already, check your mail preferences, Mail – Signature tab. It may be that the radio button is no longer selecting Plain Text.
Another possibility of fixing it, though not rigorous, is to copy your signature out to clipboard (if it is lengthy), clearing it, turning off the signature and saving your preferences. Then go back in and reset it as expected, pasting the text back in. The multi-release jump may have “confused” some of the settings. Not a great solution, but effective occasionally, with a minimum of pain.