Yep you read the title right. Luckily this happened on a test layer.
I created a very simple point layer with a related table, only a couple of text fields in each (built natively in AGOL, not published from Pro). Purpose was to be a sandbox layer for testing out Arcade expressions in Field Maps to hopefully replace a few Survey123 forms. Built a simple template in the Field Maps app page and tested a few calculated expressions (does it really not let you submit if a calculation fails? That's a no-go).
Well after submitting a point, I went into its web map viewer and clicked on "Forms" on the right-side panel. It acted like no form was built so I redid the template and saved. Soon as it went back to the web map, the point layer was throwing data errors. I haven't been able to access the feature layer since. Never seen a layer break like that before. Not a huge deal since I was just messing around, but does make me nervous about trying it in a production environment. Is there anything else that might have caused that?
@JasonJordan00 are you able to provide some steps so we can try and reproduce this? How did you create the feature layer and related table in AGOL? Do you have some examples of what Arcade expressions you authored in the Field Maps web app?
Anything to help us try and reproduce this would be helpful. thanks,