Sky light level always at 15

As can be seen in the screenshot, the F3 screen reports a sky light level of 15 during the night. This prevents hostile mobs from spawning altogether. Sky light level will decrease below 15 when standing underneath opaque blocks, as expected. I experienced this same bug in version 1.14.2, but it seemed to go away in 1.14.3, and appears to have returned with 1.14.4. I am running the latest version of optifine for 1.14.4 and no other mods.