IgbarVonSquid
Reproduction steps:
  1. Make a world with a portal to a world that doesn't have any existing public instances (Or use this one I made: https://vrchat.com/home/world/wrld_7b70203f-a8d8-4768-8ce4-71785534ae98/info)
  2. Create a Group Public instance of the world that portal leads to, park an alt/friend in there, then close the instance. By doing this, the only public instance available is a closed one.
  3. Join your test world with the portal and try to enter it. It should pop up with the error "You are not allowed to travel to that location. This instance is closed. (Code:403)"
Once this happens, the portal is "broken" and you can't attempt to enter it again unless you rejoin the world. The ideal behavior should be that the portal creates a new public instance instead of giving an error and no longer functioning.