Fooled by the sun is not the answer. I think there is a real firmware issue. I have this same problem on sunny days or cloudy days or no matter what the sky is like, flying any direction over dry lakes or clear fields or anything. The obstacle avoidance system works correctly when I'm manually flying my Mavic Pro giving me warnings when I get near real objects. I can fly out level 50 feet high over a flat open field and the obstacle avoidance system correctly detects nothing. I press return to home, it turns around, then every time it attempts to move forward it complains about an obstacle and climbs up higher, sun or no sun. It continues to do this going up hundreds of feet until I stop the RTH. The forward sensors are clean. This is not a sun issue. If it were it would complain about the sun when flying normal as well. There is something about RTH that changes the behavior of the obstacle avoidance system. This bad RTH behavior has been persistent since day one through all firmware updates.
I have to set the RTH height at 60M and disable forward object avoidance when I fly so I can have a chance to get my drone back if RTH kicks in. This is not right. I would certainly appreciate a real solution to this issue.