I have the same exact problem and this is the only post iv'e found that even talks about it so i'm guessing its a pretty rare glitch. I'm sorry i cant help you i just wanted you to know you are not alone. Also when Mirrak tries to fly away on him the first time you see him the same thing happens to him so im not sure what is going on. (the only way i could figure out how to progress is to use TCL in the console after you "stop riding" the dragon where you havent moved and just fly over to the huge pillar straight ahead from the word wall. the fight is also glitchy as hell so be prepared to quicksave a lot after sections of the fight. I hope that helps.)
