Unfortunately this is just an invalid package... someone really needs to tell the developers at Microsoft that the developers at Microsoft really want packages to be in SemVer2 format, per the Microsoft documentation
https://docs.microsoft.com/en-us/nuget/concepts/package-versioning
Microsoft's documentation seems to no longer describe what Microsoft's once documented "legacy versioning" used to be, but we captured it in our documentation before Microsoft removed it.
Bottom line, the "." is an invalid character in a non-SemVer2 (i.e. four-part version number) pre-release tag. Hence, why it's getting a 400 error. It used to do that on NuGet.org, too. I guess it doesn't now. We probably won't change this, since it kind of works already., and since it's only one package.
But if you email the Microsoft team responsible for that package, they probably will just change their versioning going forward. IT'd be nice if Microsoft documented how their non-SemVer2 packages are supposed to work.