Is citation bias a studied issue?
This problem has been receiving significant attention in recent years.  Among other thing, there was a recent scandal around "citation cartels", in which journal editorial staff organized not only to ensure their own journals self-cited but also so that groups of conspirator journals would cite one another in order to raise impact factor while adding a layer of indirection to make it harder to track.  Such journals also sometimes strong-arm authors into adding citations to other publications within the same journal.

I don't know if anybody has tried to further tease apart the different between intentional (i.e., citation cartel) and unintentional citation bias.  It would be interesting to study, but may be difficult given the amount of intentional bias that some are using the game the system.