I have experienced this in the past when a page I am working on has multiple viewports. In my case, it was always transparent enough to not cause issues, but if you get enough viewports, I could see it being more or less solid. I'd recommend checking the viewports and removing any that aren't needed. This isn't a perfect fix, but might get you on your way.
I posted how I fixed this issue previously - https://community.bluebeam.com/discussion/comment/4706?utm_source=community-share&tab=all#Comment_4706
Have you reset your Hidden Messages in order that you are informed about untitled viewports as soon as you open a drawing? I make sure I leave this feature on as I find it the quickest way to clear all of them.
We struggle with this as well because we use an integration that begins sessions for us, so we need the ability to remove viewports from within the session if they were not removed beforehand. Currently we have to download the file, remove the viewports, re-upload the file, and then launch the integration. This defeats the purpose of paying for an expensive integration.
