The support system did help me resolve my problem, but I think the UI should be tweaked to make it clear that for some destinations (and sources?) the Refresh button below the list of source fields (PostgreSQL in my case) doesn't actually refresh the list of destination fields on the right (Mailchimp in my case). I tried multiple browsers, and was about to file a support ticket, thinking "what else would that Refresh button be for, it must refresh the list of fields".
I eventually found I could clear the cache from the connection settings for Mailchimp - but this should be possible from the Mapping Definition popup.