I wanna report an issue. I can’t believe I didn’t saw it right at begining… 
Before importing my DB to Discourse I structured phpBB in Discourse style - 1 level of sub-categories max. But some sub-categories had same names. Such as:
World of Warcraft > Classes
Tera > Classes
And those sub-categories totally messed up together. I have topics from Tera > Classes in WoW > Classes category now, topics from WoW > Classes in Tera > Classes, etc. So, users beware! Names of ALL your phpBB forums has to be unique before importing.