The symbology of a hosted feature service reverts to the source symbology template during editing when accessed by a non-data owner, and the user is assigned a role, which does not include any administrative privileges.
• Create a custom role, which includes at least one Administrative privilege (for example, Members: View All) and assign the role to the user who carried out the editing task.
• Assign the desired symbology to the feature class prior to publishing, and publish the hosted feature service from ArcMap to ArcGIS Online