I have a simple relationship class between a feature class (Parcels) and a table (account information). These are existing datasets, so I couldn't use GUID or Global ID. The only common field was the address. The relate works perfectly in AGOL, but when I view it in Collector it doesn't display. I double checked to make sure the pop-ups where indeed enabled, but still nothing. If anyone would like to take a look at it the geodatabase is attached. FYI- This relate used to work in Collector. Not sure what changed.
I tested with the attached FGDB and Collector 20.1 on iOS - I can see the related records as expected. What version of Collector was this on? Is the issue still occurring?
If the issue is still happening I would recommend contacting Esri Support - let me know and I can get a case created on your behalf.