Yes, I had that happen in my Cherokee game when I vassalised Shawnee (in fact, the only time during that save where AI supported a vassal's independence). They didn't once attempt to do so, even though their own strength + support from other tribes was greater than the combined military strength I could muster, and throw in a period of several decades spent consistently with less than 10% of my manpower... :wacko:
It seems to be only there for players to utilise, to be honest.
Ah OK, thanks. That would be a shame if they do Support Independence but then never actually use it. Maybe that will be extended in future patches.