You're saying this only happens with some workbooks, and not others? Are the workbooks that exhibit this issue particularly complex?
Yes the workbooks where this happens are quite large in terms of number of tabs, but not in size (less than 1MB in some instances).
They do have data connections to our data warehouse, but previously this wasn't an issue.
It's only within the last month or two that I'm getting the image provided in my initial post.
For the sake of troubleshooting the issue, does the issue fix itself when you reduce the number of tabs, or if you remove the connections to your data warehouse?
Another thing you could try is uninstalling/reinstalling Report Builder - occasionally that does the trick as well.