Yeah, that's what kept happening to me. Just change it to:
E ZONE_CHANGE 31 1 1
It'll TP them to a corner of the map, where they're forced to town. Then, they could go hunt. I'm not sure if the 30 minute time slot exists, or if they die, they get TPed to Moradon, etc.
However, you can make a duplicate series of events to recreate your timed Forgotten Temple. Meaning, just copy their whole .EVT code, change the event numbers, PROBABLY put it on a new NPC, and maybe make a new table called Monster_Challenge_Bifrost or something. And, just make your own times for it. If you disregard that, then your Bifrost assaults will happen during your Forgotten temple.