So, i tested now a bit around with Major Expedition and Major Gallop on my Sorc, and it seems like Rapid Maneuver does NOT give you Major Expedition when you cast it on a mount, even when you unmount after the cast. It gives you Major Expedition when you cast it normally, BUT the Buff Major Expedition itself will NOT show up in any Bufflist, Ingame or Addon wise, even when its active from this Skill (if you test it you will feel the 30% Speed increase). It WILL show up from other Skills tho, for Instance Boundless Storm.
And i just guess thats a Bug. I thought, maybe it could have something to do with the Addon "Assist Rapid Riding", but i tested it without the Addon too and it didnt make a diffrence. So it has to be a bug in the game itself.
"I mustache you a question."
"Well shave it for later."