Everything is gone because when the quest is completed, the game swaps in a *new* version of the keep, i.e. the 'liberated' version. So stuff that was stashed in the old 'troll-infested' version is gone.
I believe this happens also to a few other areas.
The tip seems to be to stash stuff only after the quest concerning that area has been completed.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
