CotSG Portal in Agartha

Upon exiting CotSG into Agartha via the portal, you can’t use it to go back through into CotSG.
Returns the message:
“You need to visit the Agartha portal in City of the Sun God before you can use it for travel”

Might be related to swl’s ‘experience streamlining’ / character story line progression?

Perhaps, I thought it odd that I could pass through it - but unable to pass back through it, saying I hadn’t discovered it yet.

Yeah, it’s tied to the initial storyline of Black Sun, Red Sand on when you get there.

There should be a barrier preventing you from reaching the portal until you’ve finished Ptahmose’s first quest.

Yeah, there was no barrier then.
Can just walk right through it like it was any old portal.