Can confirm the issue exists in 16.3.6843

This isn't how message retrieval works. The autoresponder would have to be on the xxxx@gmail.com account as that's the primary address that is, essentially, receiving the message. Message retrieval simply pulls messages from one account into another as a way to aggregate accounts into a single interface. Then you'd set up SMTP Accounts so you can reply from those addresses. As it's designed, messages "retrieved" from another account are treated differently than if you email xxxx@mydomain.com directly.  (Or xxxx@gmail.com directly, for that matter.) It's similar to how SmarterTrack works, using existing POP and SMTP connections for retrieving, and sending, tickets and ticket replies.