It depends on what "will not work" means - the field is read-only, can't be focused, you can't type in the field, the new value is not available in modal.result?
I recommend inspecting the <input> element that displays the value using browser developer tools. This may give you an idea.
There may be some kind of interference with something else on your page (perhaps your global CSS). To debug that, you can try the same modal dialog in a blank page (without custom CSS and components), using the default CSS theme (without {theme: "modal_rounded"}).