“It was the Ukrainian border moving over Hungarians.”
And Hungary was part of the Axis and joined the Germans in slaughtering millions of Ukrainians.
It was entirely justified that after the war Hungary lost a chunk of territory to Ukraine, and that that territory should be home to Ukrainians.
You raise an interesting, and fair point. But it’s also fair to point out that Hungary was a somewhat reluctant ally of Nazi Germany. Some say Hungary joined the Axis out of greed, to gain territory.
Others say it was to avoid the same fate that befell Yugoslavia. (As you might know, the Yugoslavs hesitated to play ball with Hitler. So they were quickly overrun, and occupied.)
In Hungary’s case it was probably some combination of greed and self-preservation.
Anyway, I must make a gentle correction to your post. Hungary lost most of its territory after WW I, not WW II.