We have discovered a similar glitch, that is being reported as a bug. When I turn on a simple expression to calculate the current date and time in the Field Maps form, it returns the correct result when editing features with the mobile app or Map Viewer Classic. However, if you attempt to edit the same features in the same webmap with the new Map Viewer, the layer and map hang up. The work around if you need to edit features from AGOL is to open just the feature layer itself in Map Viewer, edit the features, and close. The changes will be reflected in the web map.
• ID: BUG-000149444
• Synopsis: Calculated expression in a date field for ArcGIS Field Maps renders the hosted feature services in ArcGIS Online keeps loading while editing
• Status: New