I'm thinking specifically of Bulgaria. It is well accepted that the Bulghars were conquered by the Mongols and lived under them for some time. But my boyfriend, who is Bulgarian, lent me a Bulgarian history book that had "all" of Bulgarian history and that had Roman occupation but not Mongol occupation.
In this particular case I think the problem is you're mistaking the Volga Bolghars with Bulgarians.
Because modern day Bulgarians are widely understood to be a mixture of Bulgar nobility, Slavs, Thracians, Avars, Pechenegs and Cumans.
That is one of the reasons why Volga Bulgaria (which is in modern day Chuvashia), as well as many other offshoots of the Bulgar dispersal are not looked at carefully, or at all.