What do you mean exactly by switching subscriptions? You can check the source of the tool, it works with the given subscription id, except the following case:
The tool itself logs in with azure cli as a first step, you should see the output of the login similar to below:
If the subscription here is different than you expected, that might be because the username and password that you provided belongs to multiple subscriptions, and the default is selected. If this is the case, the role assignment phase will most certainly fail.
As a workaround you can run download official azure cli and run these steps by hand after changing the default subscription.
azure account set new-default-subscription
I will file an issue to set the default subscription automatically in the tool to the one provided by the parameter.
Hope this helps!