Are u sure it's a bug? If u summon purple phantoms they cannot enter the fog. Though in very rare cases when u enter the fog before they appear they also cannot enter.

When u invade as red phantom and monsters turn on u is because they use the seed of tree giants which can be found outside firelink shrine (exit at the top just before the tower to the crow). This seed causes all monsters to attack red phantoms as well. It has a 10 percent chance to spawn whenever a person gets invaded up to 50 percent after 5 times until it appears and will reset again. Look it up.