I'm using iPhone 12 mini on iOS 14.6, Field Maps version 21.2.0 and cannot open attachments. Hosted in Enterprise 10.8.1 with hosted feature layers published by Pro 2.8.1.
Specifically in my case .pngs, I haven't tried other types. Our app is new so I am not sure if it was happening in a previous version. Here is a video of the behavior: