First of all, many thanks for adding the $latestHttpResponse variable in Mendix 7. It is a great feature and it helped me a lot when implementing response dependent error handling.
My suggestion: make the $latestHttpResponse variable also available in the parent/calling microflow. Now, the variable is empty when using a sub-microflow for the call itself. I use a separate microflow for REST calls quite often in order to use different import mappings and URL endpoints for different scenario's with a common goal.