Is this a known issue?

Yes: https://github.com/NuGet/Home/issues/4125
In the meantime, just add PrivateAssets="all" to your PackageReference item.
I understand it might be out of scope for you, but it would be nice to have info about some of these gotchas in the GitVersioning docs, since the Core/Standard/VS2017 experience is so different from Framework/VSearlier. It would save people like me from asking all these questions!
Very good point. The https://github.com/AArnott/Nerdbank.GitVersioning/blob/master/doc/nuget-acquisition.md doc hasn't been updated.
The correct syntax appears to be:
<PackageReference Include="Nerdbank.GitVersioning" Version="1.6.25">
<PrivateAssets>all</PrivateAssets>
</PackageReference>
(The comment above says PrivateAssets="all"and the doc page says PrivateAssets="true".)
Whoops. Thanks for catching. I fixed it to all instead of true. As for the child-element vs. attribute, both work with MSBuild 15.1. Did it not work for you?
I used the element, because that's what was first mentioned in the other thread you linked to. Now that I try it, I see that the attribute also works, although it's not one of the attributes suggested by IntelliSense. Makes me wonder if that usage is something undocumented that shouldn't be relied on. (The PrivateAssets element _is_ suggested by IntelliSense.)
Thanks. It definitely works by a reliable design: metadata can now be expressed as attributes. Intellisense is evidently lacking.
Most helpful comment
Thanks. It definitely works by a reliable design: metadata can now be expressed as attributes. Intellisense is evidently lacking.