While Outlook on Windows will pick up the Plain Text (TXT) email signature, when replying TXT emails, Outlook on Mac will convert the HTML to TXT.
It's is a little confusing, but this is how Microsoft designed Outlook on Mac and is out of our hands.
The result is that the TXT email signature is not the same when comparing Outlook on Windows with Outlook on Mac.
The EmaiI Signature Platform - Help Center