Hello,
You are still in GPS MODE, you are only loosing satellite signal
While on flight, when GPS signal goes under 6 satellites, lights
Turn yellow, and the phantom acts like you are in ATTI MODE, because
You do not have enough satellites to hold position. It happens to me sometimes,
But not often! It could be your location, weather, or possibly a faulty GPS MODULE?
The next time it happens, take note of how many satellites signals you have, while
Lights are yellow!
And possibly fly on a new location, to see if problem persists.
Hope this helps, and good luck, my friend!
Take care, fly safe!