My replies to Eryk's comment copied above:

Yes, always more tests :)

The behaviour of normpath has always been weird and/or incorrect around invalid UNC paths.

For example, on 3.10, normpath("//spam///eggs/..") --> "\\\\spam". Originally, the path was a file path (albeit with an invalid empty share name), and the final path is just a machine name.

Currently on 3.11, normpath("//spam///eggs/..") --> "\\\\spam\\\\". This doesn't match GetFullPathNameW, but at least it leaves the end of the path as a file (with an empty share name).

I don't think it's necessarily obvious which is correct, though matching GetFullPathNameW is certainly the easiest rule for us to use. Matching previous Python versions is also reasonable, though given the input is invalid for its domain we don't really have any obligation to preserve the result.