I am a little late commenting on this, but I too experienced an unusual drop in altitude when flying with 5.16. I set up my S800 with DJI's recommended settings. While in hover it seemed to hold fine, but as soon as I moved it around and changed attitude abruptly, it would start dropping altitude in a hurry. I could power out of it, but it was unsettling. While flying in manual mode, it flew perfectly fine. The other unusual behavior I was getting was that when I would plug in battery, one esc would initialize later than the rest. I reverted back to 5.14 and everything was back to normal for me. I have read that a few others have noticed the strange ESC out of sync. Has anyone explained this?