How are you checking for the existence of those packages? If not already can you run nuget list -source https://ci.appveyor.com/nuget/synovia-1nepli7j3g46 -pre -allversions to check for those packages.
The list running this command matches what we're seeing in Visual Studio. The list command only returns the first 1000 packages, so I ran the below command to get all of the versions of the package we're seeing that isn't actually getting uploaded. You'll see that the version shown in the screenshot isn't in this file.
It actually appears that we must be hitting a maximum allowable number of versions of a package. I've started deleting some older versions in the nuget feed and as I do so, I start seeing more recent ones pop up. So the new versions are able to get pushed up, but they then don't show up anywhere.