I think the "bug" is that you cannot upgrade VS. There should be nothing that prevents you from upgrading VS versions. Interestingly enough 16.8 isn't listed as a supported version because it wasn't an LTS release. Therefore you're running an unsupported version of VS anyway. So that is most likely why you're having a difficult time finding it. Additionally if you run the VS installer it'll auto-update to the latest version anyway.
What error do you get when you try to install Pro from the installer? I suspect it is that it cannot find it anymore. The recommended approach is to create an offline installer as documented here. This needs to be done on a machine that has the version you want though as otherwise it isn't available. I'm also going to assume that it requires that you had cached the installer files when you originally ran the setup program but I'm just guessing here.
Personally I would resolve whatever issue you're having with upgrading to the latest minor release. Nuget packages have no knowledge of VS versions (you don't even need VS for Nuget) so I'm really curious as to the issue.