It's a minor issue, with a workaround since ContainsKey works. But it did take a while to realize what was going on.
The actual bug is in RuntimeDictionary<T, S>.TryGetValue
ArcGIS Runtime 100.0
I guess my question is: Is there a better place to file a bug like this? 😉
Thanks for reporting this. I can verify that TryGetValue always returns false and I have created issue to fix it.
This will be fixed in the next release.