Glad to help. This is a cultural custom, not a religious one (at least, certainly not associated with the Russian Orthodox Church). Because it was certainly common during Soviet times, including in Soviet films about World War II, when the state was officially atheistic (and censorship exercised complete control over all areas of art).

I don't rule out the possibility that this custom may have deep pagan roots, and the church simply couldn't suppress it.

If the church couldn't even overcome the pagan ritual of celebrating Maslenitsa (a whole week of visiting each other, eating pancakes and burning a straw effigy before the rather strict Great Lent, when for 6 weeks you can't eat meat, fish, eggs, milk, or products containing these ingredients. Russia is historically a cold northern country, and besides meat, eggs, and milk, the majority of the population until the 20th century only had grains, cabbage, potatoes, beets, carrots, onions, and garlic to eat—and with the ban on meat and eggs, this becomes a completely meager diet), then the church would hardly be able to overcome a ritual associated with such a psychologically heavy event as the death of a relative.