AFAIK it’s not too complicated, basically the USSR lost territories on its western front after signing Brest-Litovsk

This isn't quite right because a lot happened after that that made the terms of Brest-Litovsk moot. In 1918 Poland became an independent country and immediately attacked Ukraine (and Belarus and Lithuania), which was in a civil war at the time. The U(krainian)SSR and the CCCP fought against the Polish and the western Ukrainian state. This war ended with the Treaty of Riga which gave a lot of Ukrainian and Belarusian territory to Poland. The Soviets got this territory back after Molotov-Ribbentrop and then WW2 happened.

The Poles still talk a lot about retaking "their" land from Ukraine and this has reached a fever pitch since Russia invaded Ukraine.