I am using Portal 10.5 and it seems I have come across a limitation. I have a polygon layer which has a one-to-many relationships to a related table. When editing, people are able to record several entries into the table against a single polygon (i.e. multiple visits made).
As long as I publish the polygon feature class and the related table from ArcMap, the relationship holds true in Portal (this is not the case when uploading the File geodatabase as a zipped file and then publishing the feature layer).
So this one-to-many relationship works in my Portal web map and also in my Portal app using a standard editing widget. It also works in Collector as long as Collector is online. But when I download a map extend in Collector to a local copy ("on device"), the polygon layer, although showing up in the layer list, is not visible in the map and can no longer be edited. If I do not have a one-to-many relationship for the polygon layer to a table, the offline version works.
I created a parallel set up in AGOL. There everything works: one-to-many editing works in the web map, in Collector online and Collector offline.
Is this a limitation between the Portal version I have (10.5) and AGOL (10.6)? Or is there a solution to this in Portal 10.5?