No issue here: you chose to override the classes in preloaded. They are there for a reason. Don't be surprised you get in undebuggable situations if you actually twist the context in which you're solving the kata.

For instance, your Or.__str__ is different from the one supposedly used.

Keep in mind that the string you see in the assertion messages are stringified versions of your CST. For instance, it seems you're returning Str(Or(...)) for that input while you're supposed to return only Or(...). So your "correct" solution is actually incorrect.

You get the discrepancy between the fixed and random tests because the supposed to be Or class is supposed to stringify itself adding parentheses around itself too. As long as your (inappropriate) definition of Or changes this behavior and that the ref solution uses your (inappropriate) classes to build the CST, the parentheses aren't there anymore in the expected result of the random tests, yes. But that's totally on you. ;)

Closing.

cheers