Altium's "Plane" support is crappy.
The shapes that will be created are limited: round or square, no ovals. As mentioned, there is no clearance checking, so you constantly get thermal relief spokes dead-ending against other spokes, and copper necks that are too thin to fabricate.
You get full functionality with a normal positive mid-layer, and a polygon pour.
However, impedance calculation only works when there is a plane present.
So it's stupid.
Fortunately, you only need to know impedance once, and calculate the proper trace width based on that. So it's not very important to use the built in controlled-impedance features.
Tim