?
There's absolutely nothing on ProGet's end that would cause symbols/source to sometimes work, and sometimes not in this manner. Async implementation in C# is very complicated and brand new, whereas source/symbols, OTOH are a 20+ year-old technology, and extremely fragile.
So, this is just a bug in VS2015, where it's not loading a remote symbol. You can verify this using something like Fiddler or Wireshark, and then see that it's not even attempting to load symbols with the async debug.
Please do send this to Microsoft, so they can fix it in a future update.