So the penal battalions, why is this happening?
This is more because of Russia's demographic issues. As I mention previously, Russia has just 8 million military aged men in a country of 144 million.
Somewhere between 500,000 to 700,000 were already in the military either directly deployed to the front or in supporting roles. 300,000 reservists were called up, that's already roughly 1/8th of the military aged men.
An equal number has fled the country to the surrounding states in Georgia, Kazakhstan, etc
So before we even start getting into the battlefield loses they suffered, somewhere between 1/8th and 1/4th of all
their available manpower is used up directly or indirectly for the war effort or fled the country.
That means Russia has roughly 6 million military aged men to do all the work in the country. All the fighting, all the production, all the transport. This includes their prison population of roughly 450,000, which I think is fair to assume is mostly men of military age.
tldr, Russia simply does not have the demographic depth to sustain a conflict like they did in WW2 with this rate of attrition.
In 1941, they had over 19 million military aged men and total population of 194 million.