The form you get when editing multiple in FieldMaps or Collector is not configurable from either the popup or the form in the webmap. Rather, it's being derived from the popup settings of the feature layer at the moment of adding the layer to the webmap. In which all visible and non editable fields are shown in the first edit multiple screen en only editable fields are shown in the second edit multiple screen. This is problematic for the following reasons:
1. It is counterintuitive and creates an unnessesary burden for users as they have to configure their popups/forms at three different places (let alone finding out that it works that way).
2. The edit form is only configurable by changing the popupsettings on the layer and removing the layer from the webmap and re add it again. Implying that symbology and all other layer settings are lost and have to be configured again.
3. This is a departure of the idea that one feature layer can figure in multiple webmaps with different popups, symbology, filters etc.
related issues are already mentioned here: https://community.esri.com/t5/arcgis-field-maps-questions/field-maps-form-not-used-when-editing-mult... and here: https://community.esri.com/t5/arcgis-online-questions/new-map-viewer-does-not-honor-field-configurat... but it's not presented as an idea as of yet.
