I ran on my shaman last night for about 3 hours and never saw any issues like the one's described. I did see a problem at one of the towers where it kept going up, turning around, going down, go up, go down, go up, go down....and I'll be uploading an error report and log on that but it's unrelated to your problem. I've got an 85 rogue. When servers come back up, I'll run him through some BG's and see if he sees the issues, since that's what you're playing.
I ran my rogue through about 4 BG's before the servers came down running Assasinate build with FPSRogue CC. I didn't see any issues. I just noticed the nice season 9 pvp gear so I'll be continueing this once the servers come back up.
Both shaman and rogue were using a combination of flying and ground mounts randomly, so not mount related I'd say.