create_categorical from unbound copy of an array is incorrect

[jamesrkg]

This works as expected:

1. Unbind categorical array
2. create_categorical using derivations referencing newly unbound variables

This doesn't work as expected:

1. Create a copy of a categorical array (using copy_variable(..., derived=False))
2. Unbind the copied categorical array
3. create_categorical using derivations referencing newly unbound variables

In the second scenario the frequencies of the derived categorical are clearly incorrect. I have examples demonstrating both of these that I can share with whoever picks up the ticket.