Based on these Descriptions, I am encountering the same or related issue using: (Telerik.Web.UI. v. 2015.2.623.45)
In describing my situation let me just out by saying, that my particular set up/UI is a little complex and I believe is contributing to the behavior which I am about to describe:
Set Up: I have 4 Grids That are all using
Each one of the grids is AJAX enabled and they are all affected/controlled by a (Customer) Selector.
Each one of the Grids has One or more validated fields for Update,
(Example for one of the columns:)
>Required" ValidationGroup="vgCustomers" Display="Dynamic"></
Validation works fine when Grids are first rendered (Unfiltered By Ajax selector), However once the Grid is (AJAX) filtered by the selector the validation does not work (looking at the rendered code confirms this the necessary mark up does not exist). Now in addition this only happens the first time around meaning if the edit is submitted (and errors) the validation reappears/works. The problem only exists when the grids reload from an AJAX post-pack initiated by the selector.
This is common to all 4 grid even though they are all set up a little differently. They also all use a _preRender method to do some specific grid customizations.
I can provide more code if necessary but do not want to overwhelm on initial Post.