Not what i would call a bug per say, more of a intentional feature.

The main issue with implementing portal retention would be in the cases of "if everyone leaves the instance no data remains of the world to have a portal in", this means if you join a world that already is empty it is effectively a refreshed instance.

This kind of synching for portals could be implemented tho in theory if the premise is that 1 person has to remain to keep the portal synchable.

But if they do that they run the risk of being left behind accidentally instead of "intentionally" so to say and even so the countdown would not reset as the last person leaving effectively terminates the portal by default.

Some further motivation towards why this needs to be even sorted/fixed would be nice, because as it is currently works fairly fine both from a security standpoint as well as social.

Dropping a portal ensures that you do not have to invite everyone and only the people currently in world is allowed into the portal, and if anyone joins late the last person in the portal can always tell them to request a invite if they supposed to come along.

To clarify, if a user drops a portal in a world, anyone who joins after will not see the portal.