The new Dashboards after upgrading (I now view it as a downgrade) to 10.9.1 Attachments will now no longer show in Details.
I filed BUG-000146647 and the official answer is that
Server-based layers and hosted feature layers from previous versions of ArcGIS Enterprise can only display attachments as a list. Hosted feature layers from ArcGIS Enterprise 10.9.1 can display attachments as a list or in a gallery.