Function call inputs do not revert to their Default Values if a variable node is connected and then disconnected

Function call inputs do not revert to their Default Values if a variable node is connected and then disconnected. Refreshing the call node does not reset the inputs to their defaults.

Reproduced in 4.7.6 binary, 4.8.3 binary, and Main (//depot/UE4/Promotable-CL-2635815)

1. Open any project
2. Create a new Actor Blueprint
3. Create a new function
4. Give the function a Float input
5. Set the Default Value of the Input to 20.0
6. Call the function in the Event Graph
7. Create a new Float variable with default value of 10.0
8. Connect the variable to the function call's input
9. Compile/Save
10. Disconnect the variable from the function call

Result:
The function call's input field reverts to 0.0 rather than the Default Value

Expected:
The function call's input field reverts to the Default Value of 20.0