At the end of World War II
You really think Poles are unaware of the reasons
why? Poland lost basically half of it's territories in the east (yes it had Belarusians, Ukrainians but also Poles mind you)
Do you think Stalin was gonna give his party colleagues a bad start in ruling over the Polish population? No. He basically gave them political ammonition to declare them more Polish (etnchnic Polish) then any other Polish politician providing with «pradawne Polskie ziemie» from the start of Poland. It fit Stalin and the Soviet Union cause it would create a lasting tension between Poland and Germany (divide and conquer tactics) and forcing Poland to rely on the Soviet Union to have her borders remain.
The border dispute is not really a problem any longer and was settled. Problem is that nowadays Polish politicians are willing to erase that border dispute in hope of gaining a monetary compensation for German warcrimes. Not thinking about the risk of opening a Pandora's box with German rightwingers.
It's shortsighted and leads only to a blind wall.
So far all there is to it, it is largely seen as a political ploy from Stalin. Not as a «gift» the guy was a cynic not a heartfelt human being.