Actually everything appeared to be simpler than that (I should blame
only me and not php mongo driver):
I was using new Mongo(); to connect to database (which is apparently
Instanciating this class will emit E_DEPRECATED warning, and turn off
Strangely enough I never saw this in my logs although I have
E_ALL as my warning level. Anyway after I changed to
recommended way new MongoClient(); the problem was solved.