When that happens, the only way I've found is to republish the layer. In my case, the contentType was set to null after I tried to do some reordering of the symbols using Manage New Features in AGOL. That is: Edit ( between the Add and Basemap menu) >> Manage >> then moving the items up or down. It is possible to change this order by updating the JSON file and avoid the contentType change. I have not been able to update the drawingInfo section of the JSON file as this also seems to set the contentType to null.
In your case, I would suggest republishing your layer. Then check the service url to see what it says for contentType. Add the layer to a new map, and before making any tweaks, see if it works both on and off-line. Then work through your map adjustments making frequent checks with the service url to see if and where the contentType is getting changed. If you can identify the step, then maybe a work around could be found.
I do have a support request in, so I may have more information later.