User is attempting to find a blueprint member variable's default value in their implementation of UK2Node::NotifyPinConnectionListChanged(). but the expected value in FBPVariableDescription is empty.
This is paraphrased. The user provides an excellent description and repro project in their case description.
User provides excellent reproduction steps in first answer:
1. Unzip BpVarDefaultValue.zip and run the editor for the BpVarDefaultValue.uproject from Visual Studio so breakpoints can be set
2. Open the Level blueprint
3. In the Event Graph, create a new variable of type Gameplay Tag
4. Compile the blueprint, and set the default value for the gameplay tag to any tag.
5. Right click in the Event Graph and search for the MyK2Node type and place it in the graph.
6. In Visual Studio, open BpVarDefaultValue\Source\BpVarDefaultValue\MyK2Node.cpp and set a breakpoint in NotifyPinConnectionListChanged()
7. Back in the Event Graph, drag the variable you created onto the MyK2Node's Gameplay Tag pin to create a Variable Get node and connect it to the Pin.
8. When the breakpoint you set in NotifyPinConnectionListChanged() is hit, step through until the FBPVariableDescription* Desc variable is set, and notice that although you set the default value of the variable in the blueprint, DefaultValue in Desc is empty
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-206350 in the post.