I'm experiencing the same exact problem and I can't find anything about this online. I'm using Kibana 6.8.1
This didn't do anything for me, and no errors appear in the Network tab in Firefox dev menu. The only outbound network request was elastic.co (there were also no console errors)
I've already set
map.includeElasticMapsService: false
but all this did was remove the road_map layer from the dropdown menu and removed the elastic.co network reqeust. Kibana seems to make no attempt to connect to my WMS server. However, in the beta Maps application, it loads perfectly fine (however I cannot use the Maps application for my use-case)