I'm having same issue, and I'm infuriated! I keep my battery fully charged and never use the Power Saver mode. I keep my light display set to 100%. But it dims to 50% light or less (even though my setting hasn't changed). Today, it's been dimming and coming back to full light *every few seconds*! This is dangerous as it could cause a seizure or cause me to kill someone. Other times, it stays dim for half an hour or more. NO REASON.
I took it off Adaptive Display mode, and it didn't help. I've tried every display mode. I'm glad to hear it's not just my phone, because 'they' always try to claim "user error". NOT! But I'm not happy anyone has to put up with this unacceptable b.s. from a top of the line, EXPENSIVE phone. I've had my Note 4 just 6 weeks, and this has been going on from the beginning. I DON'T KNOW WHAT TO DO...
PLEASE, let me know if you hear anything at all on this issue - even if it's just more complaints. I will be documenting everything... while I decide my course of action. Much thanks!