Searching for tickets with empty CF in RT 4.2.8

Hello,

I have some saved searches on my dashboard. Three of them are based on a Custom Field; one show tickets where the CF is set to X, one where CF is set to Y, and one is supposed to show tickets tagged with something other then X or Y.
To make the third search, I created the following search;
Queue = 'foo' 
AND CF.{bar} != 'X'
AND CF.[bar] != 'Y'
AND (
  Status = 'new'
 OR Status = 'open'
 OR Status = 'stalled' )

However, tickets marked with either X or Y are still appearing in the result. The same result was returned if i changed != to "NOT LIKE", and if I removed either the X or Y part.

Is this a bug, is there something wrong with our RT instance or is there something I've overlooked?
The operator 'IS' works fine, but it would be nice if I didn't have to change a query each time I updated a Custom Field..

Best regards,
Thomas Oddsund
SDS/USIT
University of Oslo
---------
RT 4.4 and RTIR training sessions, and a new workshop day! https://bestpractical.com/training
* Los Angeles - January 9-11 2017