Field Maps Designer - List contains errors

[LanaJewell]

Hello all, 

I was converting pop-ups to forms in field maps designer for several layers which are all part of the same hosted feature service. All succeeded except one, which refused to save. After some poking around, one of the fields in this form (which has a couple thousand options in a domain) showed this error which is apparently enough to block the entire form from being created:

Is there any way to view the error, either in field maps designer or out of it? It's never caused a problem being used as a pop-up, but I'm looking to add form functionality to a different aspect of this layer and can't find my way around this.