Out of curiosity...
Is this an issue that affect some clients but not others?
Does the user logging out and then back into the server resolve the reported issue?
Does it affect all users connected to the server? or some users?
Is this simply an chunk that didn't load? (I'm guessing not, but asking anyway)
Is this an issue that affects the server itself?
Is the chunk gone completely? (in memory? on the file system?)
Does restarting the server restore the chunk to the world until it is stressed enough to cause the issue again? or is it gone for good?
We don't know (that's part of why we need help!)