Because the proxy class is set to something within the game module, no warning is placed on the node as the outer package ends up being a valid Runtime module.
This should actually be a warning in the form of "Editor only node placed in a runtime blueprint"
1. Create a custom K2Node that inherits from UK2Node_BaseAsyncTask and place it in an Editor Only module
2. Set the proxy class, ProxyFactoryClass, and ProxyFactoryFunctionName to something inside of a game module
3. Place one of these custom Async nodes inside of a runtime blueprint
Current result: It compiles with a warning that the node is from an editor-only module and used in a runtime blueprint.
Expected result: There should be no warning if the ProxyFactoryClass points to a UClass in a game module. This is a valid scenario.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-112394 in the post.