i created a small mob farm at a height of 90 blocks on top of my home hill in a ex hills biome. initially i encased it in a single layer of dirt. when finished i noticed a very slow spawn rate so i went and inspected it. With no torches or any other light sources i was reading a light level of 15. i encased it in 2 more layers of dirt and the light level dropped to norm for approximately 10 minutes. then lit back up to 15. step by step not possible since issue was intermittent. but i was able to log out of my game and log back in and light had reset to norm...log out and log in again and light back to 15. i enclosed myself within a 1X2 area of dirt within the affected area and light was at constant 0. have video of issue if you like but it is larger than 10MB. once i redesigned the farm and added pistons to it the issue appears to have corrected. Possibly a bug in light level updating resolved by the repeated light updates from the pistons?