I've run into the same issue and ESRI told me that the BUG is still active. 1 of 6 layers in a Hosted Feature Layer, which were all set up similarly when the schema and map were designed in ArcGIS Pro, had default values on two fields which were not recognized by ArcGIS Field Maps. ArcGIS Online would not let me remove the default value from the field. ESRI attempted several solutions, none of which worked and they are still working on my case. ESRI's solutions were somewhat long and involved and included things like exporting to a gdb on ArcGIS online and then publishing a new layer from that gdb.
While ESRI is off doing some testing...
I figured out my own solution. The solution for me was to delete the fields in ArcGIS Online and then add them back in (also in ArcGIS online) in exactly the same way (with the same default values, same domain entries, same field type, same length, same name and same alias). If I had needed to keep the data in the fields, I would have added and calculated temporary fields to hold the current values before deleting the offending fields, then after adding the new fields, calculate them from the temporary fields, then delete the temporary fields. When I opened my maps, in which I had previously customized the layers and forms, everything was working as expected - the fields which had been replaced showed up as they should without even having to re-add them to the forms.
Farley
