Well here's something that happened to me, i'm not exactly sure why it happened but i'll explain the situation and you can mess around with it yourself.
I flew to Thorium Point.
Was mounted on top of the griffon (the griffon itself sitting in the nest near the griffon master, closest to the edge of the cliff there when my group summoned me to the Blackrock Depths using the Group Stone or w/e it's called.
When I loaded into BRM I was still mounted and I could ride through any parts of the mountain I liked, didn't dismount until I zoned into the instance.