There are things that do not work on Wayland yet

I doubt the abandoned X12 rewrite had anything to do with why a new name was introduced for Wayland. Compare Wayland to the original X11, and you'll find that they are totally different. Yes, X.org has a lot of extensions bolted on that implement parts of how a Wayland implementation works, like compositing window managers and client rendering.

Using the name X12 or X13 would probably have provoked a lot of protests, since Wayland is *not* X. There is not direct lineage from X11 code to Wayland code (Wayland isn't even an implementation, just a specification), Wayland is totally lacking the core of X11, and the X11 and Wayland protocols are totally different, and have different roles. Calling Wayland X12 or X13 is simply wrong, since it is totally incompatible with Core X11.