I got caught in the rain with my Onatah and everything's fine (only a few drops made their way to the leather). I have the moka one, that is very dark anyway, but I didn't see any spots or watermarks afterwards.
I've been caught in the rain several times since I got my leather gm (including the first day I carried it), and it is fine. The care booklet says to wipe it with a soft cloth if it gets wet, but I don't baby mine at all. It's an awesome bag!
Do you Onatah leather owners spray anything on your bag? Apple Leather Guard, perhaps? What about spraying the guard on the "strap" which could receive the most stains since it is not made of leather?