Our organization recently upgraded to ArcGIS Server (Enterprise) 10.5.1 and all of our domains disappeared only when editing in ArcGIS Online or while using the collector app. They would show up in the symbology, but not in the editor. It showed the number value, but not the text that you would expect to show.
After some unsuccessful tests re-publishing the service, and restarting/reinstalling collector, the domains still wouldn't show. I took a stab in the dark and just back into ArcMap to look at things. I had never edited the features in ArcMap because this particular feature class was built to get field data.
When I started editing in ArcMap, I realized the Feature Template for editing was not there. I created it, re-published the service, and voila! all my domains showed up again.
tldr; The feature template must be defined before you publish your service to get the domain definitions in the editing tools.