At night when undead mobs (in this case skeletons, zombies and strays only)burn when the wither is near.
To reproduce spawn skeletons and zombies at night
Watch all the zombies and skeletons burn.
Do /kill @e
Spawn a few more skeletons as the wither dies
Notice how when the wither dies the mobs stop burning soon after.
I would expect the mobs not to catch fire at night even with the wither near.