But the lat/long metadata fields filled in by using Field Maps on an iPad with the Eos Arrow seem to be in WGS84? When I bring the layer into ArcMap and calculate geometry for X and Y they are slightly off from the lat/long metadata fields, and using an online converter for WGS84 to NAD83 matches the difference. My point data is in a projected NAD83 coordinate system, with a transformation to WGS84 Web Merc applied before publishing to AGOL. The web map uses an Esri basemap so I know that the web map coordinate system is Web Merc. I believe the location profile in Field Maps is set up correctly for the Arrow (GCS ITRF 2008 -> Web Merc -> transformation based on location). Am I missing a transformation somewhere?
Have you tried setting up a field maps web map in your native projected coordinate system? I had a similar issue in the UK but set up a web map in British National Grid, that resolved the issue.
I also double checked the coordinate system coming out of my GNSS to double check the reference frame being used. But seemed to be fine. ETRF2000
Also, the transformation in use by the web map might be different to the transformation you've picked in arcmap. (You should probably switch to Pro now). To double check, you can add a web map to arcmap or Pro and then check the transformation in the data frame to compare.
Also, how much is it out by in metres?