[Bug] Cross reference to System.Threading.Tasks namespace links to System namespace

**Describe the bug**
When creating a cross reference to .NET API documentation of System.Threading.Tasks namespace (using xref), the generated link points System namespace documentation.

**To Reproduce**
I tried to recreate it by creating a new empty solution with a single project and then creating a default docfx configuration using `docfx init` and adding the .NET xrefmap.
But in the new project, the link to the namespace was generated as expected.

**Expected behavior**
The generated link should point to System.Threading.Tasks namespace documentation.

**Context:**
- OS: Endeavour OS
- Docfx version: 2.78.3+850a61c2ecb83b0bda21448f1393e480934556fd