I'll ignore your assumption that a defeated country whose capital is taken over by the enemy ceases to exist and just state the facts.
You'd better read what I said instead of deducing my assumptions.
What I said is that the state seizes to exist if there is no government
at all! The government of Poland didn't go out of the capital to the eastern part of the country. They were evacuated to London. You can't expect a foreign neighboring country to remain neutral. Of course the Red army will move in to create a buffer zone if the Polish government deserted from their duties.
When Hitler occupied Czechoslovakia in 1938 some regions of this country
in the same time were occupied by Polish troops. Poland invaded and annexed Zaolzie territory part of Southern Slovakia. Why would you expect one year later in a similar situation the Soviets to act differently?