I have modified the tests so that, even if the user's code modifies the list of customers, the validation messages refer to the original customer list. I also modified the validation procedure to give a diagnostic message that explicitly compares the number of rooms used by the user solution to the correct number of rooms.
(This refers to Python - I will do the other languages as soon as possible.)

I am traveling without access to a computer, but I think what is happening is that your code is modifying the list of customers. I added something to the description clarifying that should not be done.

Once I get home I will modify the validation procedure to provide clearer debugging messages.