Try messing with the list of actual fields that are in the popup. I don't know exactly how it worked or which kinds of fields (or maybe number of fields?) was the problem but when I removed a bunch of fields in the popup menu in map viewer and then saving to the map the popups started appearing when I tapped a feature on the mobile device. I did also reduce the number of fields in the fieldmaps designer form too so I'm not sure I can say which one of those two things did the trick. Also FWIW I don't have an ungodly number of fields on the layer, maybe 20. Maybe some of them are complicated by the fact that they are fed from a survey123 form but I still am able to have fields that are updated through Survey123 appear in a Fieldmaps popup now.
