My replies in the other thread.
Just went to check this out myself. I did experience a hiccup however after several attempts at clicking and dismissing the dialog window it popped up allowing me to purchase. something is strange though as it took a few times to get it to work
—moments later—
I am able to duplicate this! If you talk to the mount vendor from the left side (NPC’s right side) the menu won’t open. Walk away and approach from the vendor from the north or from directly in front you can open the vendor menu. if this does not work, attempt another angle after walking away briefly and coming back