If you go to AdminCP > Wiki > Maintenance > Rebuild Counters / Caches > Rebuild Page URLs, does the problem come back?
Have you ever performed an edit rollback on the affected area?
I do not see any changes since you joined here that would have caused this issue to be fixed. The last time a similar issue was fixed was about 14 months ago. If the area was created over 14 months ago, it might explain this. Or if your wiki was imported from VW3+vBulletin into VW 4.0.8 on XenForo about 2 months ago.
Unless there is another bug going on and you can reproduce the problem, perhaps it was user error, where the user actually typed 'ä'
But it seems fishy that it appeared correctly on one screenshot and not the other.