I think I may have ound the culprit (but a little concerned on what I found).
The attribute is a Plain Text standard format with a Max Length of 1000 characters. By pure guess I changed Max Length to 999 and it the filter search feature started working.
Interestingly though after I changed it and the search worked a few times, the attribute became null for all instances of that attribute. I had to reimport data...
So it appears that there is some sort of limitation when max length is over 999.