Arka and Memnos not appearing bug is triggered if one goes to the Emerald Grove waypoint area before the fight begins, usually the very edge of the cliff to pick up that one Balsam plant. Not exactly sure why it does that, but my guess is the area may be considered a part of the Grove's biome so some check gets crossed as if the player entered the Grove.

[Linked Image from i.imgur.com]

In any case I successfully haven't triggered this bug since my 1st playthrough by simply avoiding the waypoint until the fight is done.