I managed to reproduce the issue. Debugging it, I noticed two things that might have caused it.
To test the UnitOfWorkExtensions class, you don`t need to mock it in your current test method. However, if for any reason you will need this, you should specify the mock behavior to CallOriginal, like this:
This will tell the UnitOfWorkExtensions class to call the original logic if there aren`t any further arrangements. Note that, in the refactored test below, I have completely dropped the mocking of this class.
After you have specified the UnitOfWork.Current and its CorrelationId to "11111", you should expect a call to SetSubCategory with CorrelationId equals to "11111", instead of null. This is the main reason for the failing test. I wrote it like this and it behaves as expected: