This is lifted from my GeoNet post asking the same question.
I have FGDB that I am trying to build out for in-the-field surveys through AGO and Collector. Below is a mock diagram of the tables that I am having trouble. FeatureLayer1 is existing features. FeatureLayer2 is newly created features.These are simple, unattributed relationships.
In Pro: If I create a new feature in FeatureLayer2 and then a new record for SharedTable1, everything is saved without issue. If I select a feature in FeatureLayer1 and create a record in Table1 and then a SharedTable1 record, everything is saved without issue. I can even upload the edits done in Pro to AGO and they appear in Collector as expected.
In Collector: If I create a new feature in FeatureLayer2 and then a new record for SharedTable1, when I tell it to submit the record it says, "1 or more attributes failed to submit". If I select a feature in FeatureLayer1 and create a record in Table1 and then a SharedTable1 record, when I tell it to submit the record it says, "1 or more attributes failed to submit".
If I recreate the relationships, like below. All record creation is submitted and works as expected in both Pro and Collector.
Am I forgetting something rather obvious about relationship integrity in the first relationship design; is Pro ignoring something that Collector is catching? Or is there a caveat about using FGDB or Collector with this design?
Also, is there a log for Collector, either in the app or AGO that could help diagnose this problem? "1 or more attributes failed to submit" is an extremely unhelpful error message.
