The UI virtualization in RadGridView is turned on by default. This means that only the visible rows in the viewport are created. Therefore, Row count verification returns the items in the viewport. Currently, we are working on improving the behavior when working with virtualized rows but we cannot commit to any specific date when this will be addressed.
As a temporary workaround, you can try to turn off vertical virtualization (EnableRowVirtualization="False") in case your scenario allows. In this way Row count verification will return all items count.
All the best,
the Telerik team
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