Yeah, this one's common. Windows 10's own snap feature is overriding the Easy Arrange function. It's a simple toggle in the DDM settings.
Open the Dell Display Manager application. Go into the Easy Arrange settings. You'll find an option labeled "Hold down the SHIFT key to enable zone positioning." Select that checkbox. This gives Windows snapping priority over Easy Arrange.
Once that's enabled, you'll need to press and hold the Shift key on your keyboard whenever you want to use Easy Arrange to drag and snap a window into a zone. Without the Shift key, Windows 10's default snap will activate instead.