Cart completed at gateway redirect issue
Just to update this with an answer, thanks very much to P&amp;T's excellent dev team.

The model implementing RequestResponseInterface returned from the gateway's authorize() method isSuccessful() method should only return true when completing a payment.

I also had to return false on the isProcessing() method during my tests. Not sure what that method does but setting both those methods to return false fixed the issue.

It was a recent change in the commerce code that identified this bug in my gateway's code.