Whakawairua Tuahu - Unable to Orange Portal Back to First SubZone After Killing Boss

The point of this map is to clear it as quickly as possible to maximise quantity and rarity, so you're highly incentivized to not pick up any items until you're done.

However, I could not return to the 1st subzone once I killed the boss. There is no longer an orange portal to get back.

Had I thought of it, the only way around this issue is to exit the map rather than Town Portal out, and then re-enter that way. As soon as you create a Town Portal in the 2nd sub-zone, you're out of luck.