This issue is no longer reproducible in the current version of ArcGIS Online. If the issue reoccurs, contact Esri Support Services.
解决办法
Prior to publishing content to ArcGIS Online, in ArcGIS Pro run the Multipart to Singlepart Data Management Geoprocessing (gp) tool on the point feature class.
Symbolise the output feature class from above in the Firefly style.
Publish the symbolised output feature class to ArcGIS Online.