Hey psychogony,
We have done some investigating and found some inconsistencies, as you are reporting, with the Fixed Frame rate option. A bug report has been generated for this issue UE-22770, and it is currently backlogged. I cannot say when this issue will be fixed, but when a fix is implemented we will add it to our Hotfix release notes or full engine Release Notes.
With that being said, I will do what I can to add community interest to the report in order to get more priority on this issue.
Update: This has been fixed for the 4.11 release branch. You can either wait for the official release of 4.11 for the fix, or you can go to GitHub code fix and integrate it via a custom source build.
Thank you,