ViewData in PageModel.Partial is not passed correctly

When a PartialViewResult is returned via:
return '((PageModel) this).Partial(viewName: "whatEver");'

The ViewData is not passed correctly.

That is because the method calls:
'Partial(viewName, model: null);'

Which initializes a new ViewData object for the given model.
PageModel.cs line 1660:
var viewData = new ViewDataDictionary(MetadataProvider, ViewData.ModelState)
{
Model = model,
};

Instead of throwing away the PageModel.ViewData contents, i would've expected it to sustain the information in there.

Start in PageModel.cs around line 1660 and trace the PageModel.Partial(viewName) overload that returns a PartialViewResult. Compare its handling of ViewData with the expected preservation of the PageModel.ViewData contents, then verify that returning a partial without a model retains the existing ViewData.

Written by the indexing model from the issue text.