Ok, so, hear me out
I know why the allies didn't declare war on the Soviet Union and I know Russia didn't lose any war for them to return eastern Poland, however, why isn't there even a claim by the modern polish republic or a polish separatist movement in Russia? Is there?
It seems like we all agree it was a violation of Polish sovereignty, however, nothing was ever done to reverse it, or so I think. My only bet is that those regions were sparsely populated or there was a huge concentration of russians just like there were of germans in the west.
Are you referring to Kaliningrad Oblast, which is still part of the Russian Federation today? If so, that territory wasn't part of Poland before the war; it was in East Prussia, which was part of Germany (the city of Kaliningrad itself was called Königsberg prior to 1945).
If you're referring to the Kresy, the part of eastern Poland that was annexed by the Soviet Union in 1939, it wasn't part of Russia after the fall of the Soviet Union. It was distributed between Lithuania, Belarus, and Ukraine, so Russia wouldn't have been able to "return" it to Poland anyway.
The Kresy had been part of the Russian Empire since the Partitions of Poland in the late 18th century. It was assigned to the Soviet Union after World War I, based on a map proposed by Lord Curzon, a British diplomat, which gave the Polish-Soviet border the name "Curzon Line". However, after the Polish-Soviet War in 1919-1921, the Treaty of Riga gave the Kresy to Poland. The Soviet Union occupied and annexed the Kresy in September 1939, after they joined in the invasion of Poland under the Molotov-Ribbentrop Pact. It was occupied by the Germans in the opening days of the invasion of the Soviet Union in 1941, but was mostly liberated by the Soviets by the end of 1944.
Even before the Soviets had retaken the area, at the Tehran Conference in 1943, Stalin demanded that the postwar border between the Soviet Union and Poland be set at the Curzon Line. He reaffirmed this demand at the Yalta Conference in 1945 and later that year at Potsdam, the Allies agreed to the postwar border proposed by the Soviets. The Soviet-backed government in Poland agreed to cede the area east of the Curzon Line to the USSR; as compensation, they received territory in the west that had been part of Germany prior to the war (the western Polish border was moved to the Oder-Niesse line, where it remains today). This territory was smaller than the Kresy, but it was more economically and industrially developed, and included the major cities of Szczecin (Stettin), Gdansk (Danzig), and Wroclaw (Breslau). Poles were expelled from the Kresy by the Soviets, while Germans were expelled from the new western territories by Poland.
The territory in the Kresy was divided between the Lithuanian, Byelorussian, and Ukrainian SSRs, and after the breakup of the Soviet Union, became part of the independent Lithuania, Belarus, and Ukraine. Poland had no real claim to those territories at that point, and in any case, Russia wouldn't have been involved in any such claim.
Source: Piotr Eberhardt, Political Migrations on Polish Territories, 1939-1950 (Polish Academy of Sciences, 2011)