Run the attached sample project on a monitor with 100% DPI scaling. Then, try moving the form to the second monitor with 150% DPI scaling. You will notice that the following error occurs:

Workaround: instead of using a RadTextBoxControlElement in the custom cell, feel free to use a RadTextBoxElement. However, please have in mind that RadTextBoxElement hosts the MS TextBox and using controls in grid cells may slow down the scrolling and will cause visual glitches as they do not support clipping.