So you've had cases where NO userid was set, and others where the WRONG userid was set?
Wow.
Sounds like a problem with reading or writing to your custom table.
What is the key of this table? The workitem id, I presume?
In those cases where bad userid was set, can you still see the (wrong) entry in the Z table?
If so, it's a writing issue. Otherwise, it's a reading issue.
Finally, can you trust what the users reported in these rare cases? Was the blank/wrong userid in fact entered by them?
good luck :-)
Paul
PS I don't usually agree with designs where the users choose their own approvers.
It opens the door to collusion and/or may slow down the approval process.