When F7 is pressed, it shows Xs on the ground to show where mobs are able to spawn, and getting rid of the X should make it so mobs can no longer spawn, except mobs are STILL spawning, and in the location where they do, the Block Light is 7, but no X has appeared. (I don't know what mod it is that gives this capability, but would updating it possibly fix the issue?)
I just watched a Zombie spawn next to the spawner, when there were no Yellow or Red Xs.