Version:           2.0.1 (using KDE 4.2.1)
Compiler:            i686-pc-linux-gnu-gcc 
OS:                Linux
Installed from:    Gentoo Packages

Several files from the collection - for example from 4 Non Blondes Album 'Bigger, Better, Faster, More!', track 1 'Train' did not appear in the collection.
I updated the tags outside of amarok, assuming they were missing.
In amaroks settings I triggered rescanning the collection. After the rescan was finished, the missing track appeared in the collection viewer.
Several seconds later it automagically disappeared again. 
The same with tracks 5 and 6 from Alanis Morrissetes 'So called chaos' (and probably more)

Rather annoying.

I can drag the missing files from Dolphin into the playlist and they are played, Album thumbnail is missing though.
If I do the rescan via settings again, the track reappears in the collection view only to disappear again after a couple of seconds.