during a amirdrasil run I could not mount after tindral dropped feathers and thus could not enter where Fyrakk is. I got a feather but after that mount was not possible, it was greyed out. I tried dying to get to fly but did nothing.
You have to be in skyriding mode to be able to use the feathers and fly to the next platform.
yeah I found out, you have to be in skyriding mode BEFORE the feathers, so before tindral fight make sure to be in right mode.