I noticed when replying to emails that were sent in plain text vs. HTML that my default signature would include the name of the signature above the actual configured signature in the reply.  In my case, “Adam” was the name of the signature I created for my GMAIL signature.  In the images below you see my signature and Adam above it.

Go to Outlook>Preferences>Composing and uncheck “When replying or forwarding, use the format of the original message.” which seemed to resolve the issue.