I'm using ArcGIS Pro 2.9.0 and have several feature layers piped in from our ArcGIS Online Portal. In the beginning, everything was housed in a geodatabase where attachments were enabled and added. Those same layers were then uploaded into AGOL where we were able to view and (until recently, in one particular layer) edit the attachments.
Then, somewhere along the way, something happened. I've no idea what it could have been.
I am still able to view the attachments for that layer. When I go to Properties -> Source, it does not say anything like it did before ("contains attachments") about attachments. And when I go to its Attribute table and right click on a single record, the "manage attachments" option is grayed out and I am unable to select it.
Have you tried using the Enable Attachments (Data Management) geoprocessing tool in ArcGIS Pro to ensure attachments are enabled for the geodatabase or feature class?