I noticed this in one of my own scripts, but I can reproduce in Spezial:ApiSandbox, too: When you use an OO.ui.DropdownWidget before a mw.widgets.datetime.DateTimeInputWidget, the date will overlap the dropdown list.
See the screenshot, or go to Special:ApiSandbox, select action=query and list=allrevisions, then enable arvstart, and open the list for arvnamespace.