I wanted to update this thread after getting some confirmation from our development team.
In short, the GPS map widget should not have been available through the MQTT API despite it being listed in our documentation as an accepted data type (and our sorting our the correct format with which to send GPS data). As mentioned above it was designed for LoRa devices and needs further development to work properly with non-LoRa devices like those connected via MQTT.
We're going to hide it from our documentation until such time as it is fully developed and tested. Until that time, you can probably still create a GPS widget by publishing gps,m=[lat,long,alt] type to Cayenne, but I wouldn't expect it to work/retain data.