Ah this might be something I can actually help with haha, in my budgie install where I have 4 monitors in a square grid I am unable to drag "through" a panel so maybe try the following:
Do you have a top panel? Try moving your top panel to bottom/left/right and see if that lets you drag a window to your top screen
Alternatively you could make your top monitor the primary display and make sure you dont have a bottom panel (i dont know your setup)
Budgie has some small quirks like that I hope will be fixed in budgie 11 since If I remember correctly I think it was mentioned it would have either better multi monitor support or better panel / multipanel support but dont quote me on that
