I've not seen the movie so I can't specifically comment on what it portrays but there's a couple of points that would be relevant:

• The soviets are confirmed to have recruited korean soldiers, both ex-japanese POWs and koreans from the far east region, into their army. This goes back to tsarist times.

• Far east koreans were forced to migrate en-masse to central asia because they might act as a fifth column for the japanese (this is quite ironic). A significant proportion died en-route and on-arrival so we can conclude that they didn't regard them on the same level as russian soldiers.

Put those two together and it's entirely possible that they wouldn't hesitate to use them as cannon fodder.

On the other hand there weren't that many POWs in the first place so I doubt that this would've been a deliberate or official policy so much as a decision made locally.