Your dateString is of date or timestamp data type with a numerical representation, right? (You could check this in table view). It seems that the right function is applied to the numerical representation, not the textual.
But I would agree this is not the result I would expect and it seems to break existing coding.
I am just working on QV 11 IR at the moment, in this version it's returning the correct string part.
I believe your dateString will automatically be parsed in as date type by QV (given the standard date format matches). You might want to check the date type in table view by hovering over the field to verify this.
And I believe Martin's suggested workaround should work also, but using date functions like Month and Day might be a good solution anyway.