Hostile mobs in daylight

Random dark spots are appearing during daylight hours. These are dark enough for hostile mobs to spawn. Even after leaving the spot, the mob is not catching fire as usual. Approaching the spot causes it to refresh the lighting level and it disappears.

SS 1: Zombie in full daylight
SS 2: Dark spot at range in crosshair
SS 3: Same spot after being approached
SS 4: Angle to sun from spot location