[Derision]

This looks to me to be a bug in the source code in QuestManager:pvp

The code is indented like this

        if (!strcasecmp(mode,"on"))
                if (initiator)
                        initiator->SetPVP(true);
        else
                if (initiator)
                        initiator->SetPVP(false);

But I believe it is being inteprereted like this

        if (!strcasecmp(mode,"on"))
        {
                if (initiator)
                        initiator->SetPVP(true);
                else
                        if (initiator)
                              initiator->SetPVP(false);
        }

In the meantime, rather than using quest::pvp, you could use $client->SetPVP(toggle), so $client->SetPVP(1) to turn it on and $client->SetPVP(0) to turn the flag off.