I have three ATV 3s that have been doing the same thing for several months across multiple firmware versions. All three TVs are 1080p, but I intentionally set the ATVs to 720p for casual viewing to reduce the amount of data used by Netflix in order to avoid surpassing my Internet provider's monthly data allowance.
Some time ago, I began to notice that the video resolution setting reverted to "Auto" from "720p - 60 Hz". First on one ATV then another. Initially, I thought I was crazy and that I had just forgot to change the setting. Then, I was convinced I had changed it, but it woudn't "stick". I concluded it was happening each time the firmware was upgraded. I never cycle the power on any of my ATVs - I just let them go to sleep. Recently, I noticed the video resolution setting was reverting to "Auto" and setting itself back to the 1080p capability of my TV without a power cycle or firmware upgrade - it just did it "all on its own".
This is a very frustrating problem to have to check and sometimes adjust the video setting every time I watch Netflix on one of my ATVs!
Here's someone else with the problem...
https://discussions.apple.com/thread/4667592