@molni576 Oh yeah, I was referring to that "Add" option underneath "What kind of editing is allowed", so that all looks normal.... hmmmm.
After quickly skimming through the tutorial that you followed, I noticed that his videos are 2 years old and there have been some significant changes since then. He was creating his web map in Map Viewer Classic, then configuring the form in the separate Field Maps web application. I don't know why this wouldn't still work now, but I would maybe try approaching it from a different workflow -
- Try creating a new web map in the new Map Viewer (not Map Viewer Classic).
- Add your feature service that contains the parent and related table.
- Configure the related table form directly in this web map.
- Try it in Field Maps on your mobile device and see what happens, if you get the same behavior.