I have exactly the same problem. But mine is different, sometimes I open a project, and all the Dynamic Attachments are visible. But, the next day, I open the same project, and now there gone. I guess it has something to do the video. Bit not that I don't have good video, I have a 8 GB Nvidia Quadro P4000. I'm referring that ArcGIS Pro, like most of the time, is not responding right with the computer hardware. And by the way, I'm using ArcGIS Pro v2.5
I was using ArcGIS Pro v2.6 but had to go back to 2.5 also because of a huge Bug exporting Map Series PDF.
So, I don't know what to tell you other than try to find out if there is some kind of trouble shooting to address this issue. Lets see if somebody from ESRI staff replies...