I'm not exactly sure what the question is referring to, as Prussia never lost formerly Polish lands acquired during the Partitions of Poland, as they remained a part of Prussian territory until the formation of the Second German Empire in 1871, when they became a part of Germany and were ultimately lost by Second German Empire after the restitution of Poland in 1918 and later by Nazi Germany (Third Reich) in 1945, in the wake of the Potsdam and Yalta Conference.

You may be interested in the brief summary of the history of relation between Prussia and Poland here and the reasons for moving the borders of Poland west in 1945 in this recent answer.

After 1917, leaders of the Entente agreed that the existence of Polish sovereign state is beneficial to the situation in Europe as it would have both impacted the status of the Central Powers and provide the buffer zone between the Western Europe and the communist Russia in case the revolutionaries eventually won the Civil War, what they did. Thus, after the Entente victory in 1918, the resurrection of Poland was a foregone conclusion, especially given that Poland formally declared independence through the decision of the Regency Council in October that year, although a precise delineation of the borders of this country has been postponed and took place during the Versailles Conference in the years 1919-1920. The border with Germany has been established as a part of the Peace Treaty with Germany, signed on 28th June 1919 that granted Poland a Pomerelian territory west of the upper part of Vistula River (but without Danzig that became a Free City under the protection of the League of Nations) and majority of Greater Poland in the wake of the successful national uprisings in the region. In the parts of Eastern Prussia inhabited by substantial number of Poles a plebiscite ended with the majority of the votes being cast in favour of remaining on the Prussian side of the border.

Thus, Germany lost a part of its land to the newly reconstituted Poland with the lands in question largely belonging to Prussia prior to German Unification of 1871. The reasons for the loss of the other parts of former Prussian territory, chiefly East Prussia, Lower Silesia and Pomerania in 1945 are described in the second link presented above, so I don't think it is prudent to repeat it here.