[Radek]

I think it's the combo Create Portal - Teleport which is buggy somehow. Let us note that the Teleport spell is the one that raises most of complaints. And the complaints are always the same: "I have set the portal but my teleport fizzles! Why?". This happens also in the wilderness (often behind the shrine) where some "good portal so far" should be supposed. The teleport does not teleport to the old portal - it fizzles instead.
Therefore, the Create Portal seems to be buggy. Not only you are mislead by the "portal created" message, the spell also deletes your current portal without replacing it by a new one. This must be a bug: if the Create Portal spell fizzled simply and lied that a portal was created for some reason, then the Teleport spell would have to teleport to the old portal. This is not the case. The complaints from dungeons can fall into the same category but we don't know whether a "good portal" has been created so far.
The only defense against the bug is keeping the bug in your mind. If you are far from the town or if it is essential that you have a portal at a certain location then always check the portal by a subsequent Teleport spell. Create the portal, save the game, make a few steps away, and teleport. If you get teleported to your new portal then the portal is safe. It will not vanish (until you create another portal) and all Teleport spells will teleport you to your new portal.