I just tried to recreate this by going back to my save between the fight with the goblins at the gate and meeting Zevlor and going through the exact same dialog options, and I've managed to recreate the problem. If you rest, travel to camp, or even just reload the save (after you speak to Zevlor the first time & the second time - once in the pre-fight thing with Aradin, and once after) so that Arka runs past you yelling about her brother, that little bit seems to trigger Zevlor's dialog breaking and reverting to "find me in the caves".
I don't know if it's Arka running past or resting/travelling to camp/reloading the save that triggers it though, because doing any of those three things causes Arka to run by yelling about "that bloody goblin", which seems to be the indicator that you WILL encounter this bug.