The bug

Wither bosses can shoot blue wither skull if no enemy is around and there is enough space. The value if a wither skull is blue or not is not saved in the NBT data which means it loses this after reloading it, you cannot summon them in and you cannot test for them.

The reason

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

The reason why this happens is because the field net.minecraft.entity.projectile.EntityWitherSkull.INVULNERABLE is not written to the NBT data and cannot be set based on the NBT data.

Note: A tag for this field should not be named "Invulnerable" as this value is only used for less destroyed blocks when it explodes and for rendering. The behaviour of the "Invulnerable" tag is not affected by this.