I am having a similar issue. I have a points feature class and a relational table for condition reports. I would like to be able to take pictures as part of my condition reports.
When I publish the web service to my Enterprise Portal as a Global-ID-enabled relationship without enabling attachments, my relational data shows up in the Portal, I can view relational data in a web map, and I can add relational data via Collector.
If I create a relationship between my feature class and my relational table, then enable attachments on the relational table, it breaks the relationship between the feature class and the relational table.
If I enable attachments on the relational table first, then create the relationship with my feature class, it seems to work fine, and shows up both in the Portal and in Collector. My pictures are being captured related to the reports, and they show up in Collector and ArcGIS Pro, but I'm not seeing them in Portal.