Zah'ran in range for interaction but not vendor access

On my Haranir druid I approach Zah'ran at the end of The Darkway delve, about a yard away from him. My mouse pointer vendor bag icon turns from grey to gold, indicating I'm in range. Clicking on the NPC displays his dialog text and the 'Show me' vendor button. However, at this same distance clicking this button gives me the error 'You need to be closer to interact with that target' and closes the dialog window. I can, of course, move even closer to the NPC and then vendor access works fine. But it seems like a bug that his dialog interaction distance is greater than his vendor access distance, especially when the mouse pointer indicates vendor access distance has been reached.