Sorting a Blueprint variable into a subcategory separates variables into separate category structures

Within the Blueprint Editor, it is possible to create subcategories for variables to be sorted into.
Doing so separates variables not nested into subcategories out into standalone, top-level structures.

I can reproduce this issue as far back as the live 4.20.3 Binary (CL 4377374); it is therefore not marked as a regression.

1. Open any project in UE4Editor
2. Create a new Actor Blueprint
3. Within the Actor Blueprint, create a new variable called Test
4. Set Test's Category to Stuff
5. Within the Actor Blueprint, create a new variable called Test2
6. Set Test2's Category to Stuff | Things
7. Compile and Save the Actor Blueprint
8. Drag an instance of the Actor Blueprint into the Viewport

Result
In the Details Panel, the following sections and variables are present

• Stuff
  • Test
• Stuff
  • Things
    • Test2

Expected Result:
Variables created with the same root category are combined, example:

• Stuff
  • Test
  • Things
    • Test2