There are a handful of topics from a few weeks ago talking about this issue, but even with the 1.04 beta patch there is no mention of this. I followed some advice when this issue first happened when I revisited Caed Nua, and was able to get Kana back in my party by restarting the game. However since clearing to level 8 of the Endless Path and doing part of his quest, I can't return back to the surface without him disappearing from my party.
If I go into the Keep and come back out, he appears on the map for some reason way up on top of the Keep where you can't even get to.
Here is the save game right before trying to return to the surface from Level 8, along with the Log file and screenshot of Kana on the map.
I hope this issue is still being looked at because I can't continue my game until it's fixed. I could I guess continue the game without Kana, but something tells me I won't be able to fix it if I continue the game with him removed from my party with this bug.