Client has reported that sometimes point attachments will display the wrong attachment/s in pop-ups. They have been able to capture this issue with video screen capture. We have now also been able to replicate this issue. So we believe this issue to be "real" and not based in user error.
In detail this is what is happening; a user is selecting power poles in a ArcGIS Portal Web Map (10.8) and reviewing their photo attachments. As the user works a long a chain of poles they will suddenly get a pole that actually links the user to a photo attachment for a previous pole. If you keep selecting poles and looking at photo attachments, then eventually the pole that was displaying the incorrect pole will correct itself.
This of course is extremely disruptive and will require more investigation, looking to submit this as an official support ticket very soon. One supplementary note is that we were only able to replicate this issue in FireFox web browser so far, so have advised client (who recently changed to FireFox) to change back to using Chrome to see if this is more reliable.
Interested to hear if anyone else has experienced this issue in their own environments before I submit an actual support query to our provider/reseller?