Light bug in the mob grinder

In the debug screen (opened by F3 by default; unless your server has deprecated debug info) there is part about light, you can see blosk light and day light in there. Note that daylight isn't current level of light due to sun but level of light during the day. Also note that both figures speak about the block containing your feet. Walk in the lighted part and observe the value. If it dispalys zero, then you are experiencing a graphical bug that should not affect mob spawning. This can happen (or at least could in earlier versions) for example if the floor is made out of upper slabs or upside down stairs that are lit from the other side.

If you see an actual number, there probably is an actual light source you overlooked. You can follow the direction in which those numbers would grow to locate it. It might help with diagnosis to open your graphical settings and turn smooth lighting off.

If you locate the spot with maximal light, but still do not see a light source, try placing something there with hopes the game updates. If not, you have probably found an actual bug. Be sure to report this to Mojang.