Data added to ArcGIS Online as an item (CSV file containing addresses) has a different default style than the same data published from ArcMap.
Last Published: September 11, 2015ArcGIS Online
Bug ID Number
June 4, 2015
August 21, 2020
Will Not Be Addressed
The development team has considered the issue or request and concluded it will not be addressed. The issue's Additional Information section may contain further explanation.
This is expected behavior.
The real issue here is that data published from ArcMap has a different default style than data added as an item to My Content folder in ArcGIS Online.
With an added CSV, the default style symbol for a point is a 3D symbol that cannot have the fill or outline modified. Change the symbol to the basic point to modify the fill and outline.