Mob vision is shared with living players within exactly 94 blocks of a Demon possessing a Mob with a filtered vision. This was tested with a Creeper on the exact same Y and Z plane as the living player. Mob vision goes away if the Demon ejects from the Mob and doesn't affect the living as long as they are 94 blocks away. Going within 94 blocks will cause the bug and the effect will stay for as long as the Demon is possessing, no matter how far away. The only fix is to eject and re-possess further away.

I don't have any errors in console but hopefully this can give enough info to help sort this out.