Hello,
same isuse here, but I cannot confirm that id did not work in ARB 5.0 at all.
End of July, I refreshed my reports with ARB 5.0.27 and all Bounce Rate metrics were OK. Then I refreshed again end of August (still with ARB 5.0.27): all values were wrong.
After update to ARB 5.0.44, the problem was solved.
Neither the existence nor the fix of the issue has been announced in the Release Notes. I would expect that releases do fix known issues, but not "implement" bugs to older versions. It is not always possible for users to update to the latest version of software, due to internal software release processes etc. Furthermore, it is not possible to check all reports for data integrety after a update.
Therefore, it would be great if Adobe not only would inform when bugs are fixed, but also if releases or new versions influence older versions or make older versions not running well.