If this isn't an issue for hackers, then I'd like to remove it for now. So that brings up the question, how?
I would love to see a fix for this very very soon.
I fixed this issue by replacing each of the individual files affected by the update with those from my pre-upgrade version of phpbb. I didn't have a backup of my forum before the update so I just downloaded 2.0.21 and used the files inside to replace those affected by the upgrade. My board still thinks that it has been updated and the issue has went away!
Here's a list of the files which must be replaced to fix the problem.
<--- This might be the only file you need to restore. If so, then your good to go. If not, then replace them all.
Hope this helps.