When using the AddAjaxSettings method of the RadAjaxManager, the Height is not taken into account. 

In the following example, the "Unit.Percentage(100)" part marked with yellow:

RadAjaxManager1.AjaxSettings.AddAjaxSetting(RadGrid1, RadGrid2, RadAjaxLoadingPanel1, UpdatePanelRenderMode.Block, Unit.Percentage(100));

This worked with older versions of Telerik, e.g. 2020.x.y