The sole reason Ukraine was a founding member of the U.N. was due to Stalin's demand that all 16 republics (Including the Krealo-Finnish SSR which was created solely to facilitate a annexation of Finland) be given seats in addition to the seat for the USSR itself on the grounds they were nominally sovereign states, especially because at the founding the UN there were only 5 communist states in the world and they wanted to balance out the votes in the general assembly
The U.S. was fiercely opposed to this making a counter demand for all 48 states to be given seats on the same grounds due to them being sovereign states under the 10th amendment, but the British convinced the U.S. to compromise to allowing 2 Soviet Republics to join so the USSR wouldn't block membership of Canada, India and New Zealand which were still under the sovereignty of the United Kingdom, supposedly the U.S. was given the option of granting seats to two states but it was never acted on.
Also let me remind you who was the leader of the Ukrainian SSR in 1945 at the founding of the UN: It was Nikita Khrushchev, who is a ethnic Russian, born in Russia, and lived in Russia until he joined the Red Army during the revolution, worked at a mine in Donetsk, then moved to Moscow in 1929 and became secretary of Moscow Oblast in 1934 directly under Stalin, carrying out the great purges by ordering the execution of thousands of people, he was so good at executions that Stalin personally appointed him head of the Communist Party of Ukraine in 1937 and within 3 months of arriving in Kiev he had ordered the execution of so many members of the Ukrainian Communist party that their Central Committee couldn't even meet quorum with nearly every Political Official and Military Commander in Ukraine arrested and/or executed.
But yeah man Ukraine was a founding member of the U.N.