What is the ruling on forgetting to complete 2nd wash of the face in wudu for sunnah then, after washing only half of the right arm, you return back to complete the sunnah washes of the face then carry on as normal

There is no problem even if u did not go back because only the 1st wash is mandatory, but it has to be correct and covers the whole area and it is always better to wash 3 times to make sure that it is correct and covers the whole area.