Filter - Contains not working

Hi,
I have a radgrid.  Within this I have enabled filtering.   I have set the filter type to Combined. 
In the gridboundcolumn I have specified ‘Contains’ as the CurrentFilterFunction.  However despite this when I filter the column I only ever get rows returned that exactly match the filter, so I presume it’s enforcing an ‘EqualsTo’ filter.
Is this by design or a bug?

Thanks.