They escaped from the Germans, the Soviets put them in the gulag. Eventually, the Czechoslovaks fell at the front
80 years ago, the Battle of Kiev took place as part of the great Soviet offensive during World War II. Czechoslovak soldiers also took part in the fighting near the Ukrainian capital. They fled to the Soviet Union to escape the Nazis, but ended up in the Gulag. When the Soviets released them after the amnesty, they formed the first Czechoslovak unit, commanded by the later President Ludvík Svoboda. Over thirty of their compatriots, mostly from Subcarpathian Rus, were killed in the Battle of Kiev. Some of the soldiers died of non-combat causes, for example, when they poisoned themselves in the town of Vasilkov in the Kiev region with a liquid they thought was alcohol.
Source https://aktualne.cz/domaci/galerie-kyjev-cechoslovaci/