I am in my singleplayer world, and I have my debug menu open. I was looking at the light level while walking through my strip mine, and for a single frame, the server light changed to something I couldn't really see. I recorded it and slowed it down and saw that it changed to "??" for a single frame.

What worked for reproducing it was creating a world with the same seed and teleporting to the coordinates where the issue occurred. I flew into the area at the lowest spectator mode speed, and it happened at the same location (between Z coordinate 79.999 and Z coordinate 80.000) and it happened. I decided to randomly fly around the area and noticed that it actually happens all the time in a ton of different places. So I created a new world, and the same thing happened, it still messed up the server light all the time in random places.

TL;DR: Create a world (preferably in creative mode) and fly around, look at "Server Light" in the debug menu.

I tested this on a vanilla server and the glitch wasn't present.