USS variable from another USS is not listed in the Set Variable picker when styling a selector in a different USS on the same UXML

How to reproduce:

1. Open the attached “IN-147834” project
2. Open Test.uxml in UI Builder
3. In the StyleSheets panel, select the Style2.uss > .test-class
4. In the UI Builder Inspector, locate Background > Color
5. Right-click Color > Set Variable and scroll down
6. Observe the variable list

Actual result: -my-color (defined in Style1.uss) is not listed - only the built-in --unity* variables appear
Expected result: --my-color is listed, the same way it is when styling a selector in Style1.uss or the element inline

Reproducible with: 6000.0.0f1, 6000.0.80f1, 6000.3.20f1, 6000.5.5f1, 6000.6.0b5, 6000.7.0a3

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Workaround: Type the variable name manually into the Set Variable field instead of selecting it from the list