For what its worth for those who stumble on this thread. We had the same issue (Portal 10.6.1) and realized it was a very easy fix. From what we have found, you need to use a Feature service, not the Map image service. When we first published this service we figured a Map Image Service alone would suffice.
However...when loading in a map image service to the Map Viewer, the option for heat map display does not show up. You can still filter and change symbology etc.
When loading in the same underlying source but as the feature service, you will see the option for Heat Map symbolizing.
So we republished our service with both Feature and Map enabled and it now allows for Heat Map symbolizing.
