Hi Stefan, that's basically a bug, it's related to your input curves being particularly small in size.

The generated plane is within tolerance at such a small size, but if you were to scale the object up it would be problematic because the underlying surface is just a simple 4 point plane but the edges of its trimming boundaries would be quite a big distance away from it, probably making poor results if they were used in some other kinds of calculations after that.

So it's probably not something to rely on, and you probably should scale up your curves so they're not so teeny-tiny before running Planar on them.

The Planar command is supposed to be using a size-relative tolerance for determining planarity but it looks like something is ending up using an absolute tolerance instead.

If things were working as intended you would not actually get a surface generated on that one fairly non-planar loop.

- Michael