I don't know the answer but here's something to consider. Have you tried switching to "A custom attribute display" with the MapViewer Classic pop-up settings or using Form Builder in Map Viewer?
I agree with you about the default area being displayed in Field Maps. Many times it is not helpful.
And with points, it shows coordinate, and lines it shows length.
Been looking for a way to get rid of that for some time, no luck. Would be nice to be able to configure a popup to only show the info you want it to in Field Maps (like in both map viewers).
R_
never noticed this feature before but I do see it in one of my Field Maps apps now. Curious to see if anyone has a solution.