Thank you for contacting us regarding this problem.
It seems all the columns are exported even if you have Display="false" setting. It doesn't seem related to the IgnorePaging feature. You can still use Visible="false" instead.
I have notified our developers and the issue will be fixed shortly. I hope the workaround is applicable for you meanwhile. I have also updated your Telerik points for notifying us about this problem.
the Telerik team