Why does nuget consider 1.2.3.4 to be a stable (release) version?

I'm seeing below:

1.2.3          // release1.2.3+hash     // release1.2.3-alpha    // pre-release1.2.3-alpha.12 // pre-release1.2.3.4        // why is this release version and not pre-release version or invalid?1.2.3.4.5      // invalid nuget version

I don't see why 1.2.3.4 is considered as a release version mentioned anywhere below:

• https://docs.microsoft.com/en-us/nuget/concepts/package-versioning
• https://semver.org/