Not sure but there are a few bug reports somewhat related to what you're seeing:
BUG-000111131 : If a time zone is set on a feature service in ArcGIS Server, if edits are not made on the date field and the date is null, the date field will auto-populate with 12/30/1899.
BUG-000097813 : When setting a time zone on a map service in ArcGIS Server, if edits are not made in an ArcGIS Online web map to the date field and the date field is blank, the date field will auto-populate with 12/30/1899.
There were some notes internally about the issue occurring when a hosted feature layer consisting of a date field is republished and then again exported as a file geodatabase from ArcGIS Online/Portal. Also trying to view the fields to change the default value will throw the error. It was observed in ArcGIS Pro 2.4.2 and ArcGIS Pro 2.5 (Alpha 3). Those bugs are to be fixed at 2.8 it seems.