Attachments are shared among the sub-layers within a hosted feature layer if the same feature class is added multiple times in a map while publishing to ArcGIS Online.

Workaround

Use one of the following workarounds:

• Create multiple hosted feature views in ArcGIS Online > ‘Show’ or ‘Hide’ the attachments from their item details page. 
• Create multiple feature classes in the file geodatabase, and then publish it to ArcGIS Online as a hosted feature layer.

Steps to Reproduce