When trying to batch edit cell values of the Grid via e.g., RadEditor (GridHTMLEditorColumn or RadEditor in EditItemTemplate of GridTemplateColumn) the cell value always result to a plain text. 

The issue will not be fixed since there is a big chance to introduce a breaking change. Nevertheless, there is a simple solution that could be use. The solution is to manually remove the hidden column from the ClientState on the client.

    <script type="text/javascript">
        function test() {
            var gridID = $telerik.$(".RadGrid")[0].id;
            var clientState = document.getElementById(gridID + "_ClientState");
            var value = clientState.value;
            if (value) {
                var parsedValue = JSON.parse(value);
                parsedValue.hidedColumns = [];
                clientState.value = JSON.stringify(parsedValue);
            }
        }
    </script> 

The manually entered value of RadDateTimePicker is replaced with the original one in Batch Edid mode in iPad.

To reproduce this incorrect behavior add a new 

<telerik:GridDateTimeColumn HeaderText="Start Date" UniqueName="StartDate" DataField="StartDate" DataFormatString="{0:MM/dd/yyyy}">
            </telerik:GridDateTimeColumn>

column in the Batch Edit demos of RadGrid and test under iPad. 

Tap over RadTextBox, enter some new date and press Go/tap out and you'll see the problem.