Rebelling faction members getting volunteer troops isn't the problem, no. Those volunteering peasants coming in ludicrous numbers of heavy infantry that somehow manage to survive in the wilderness as well as the Mongols? That is your bug right there.
It still seems like it's less of a bug and more of an oversight about the consequences of almost every faction getting these. It works well enough for smaller realms but when combined with the fact that the rebel doomstacks steamrolling 5-10 provinces is a victory it makes putting down even small revolts impossible for a large empire because that can be achieved before a large empire has a chance to marshal its forces. Clearly it needs to be changed, but I see no reason to call it a bug.