My USB-C extension cable has this helpful label stuck to it:- "If there is a problem when connected with your mouse, keyboard, etc (USB 2.0 devices can only support one side), please reverse the female connector of the USB-C extension cable" USB-C connectors are symmetrical, but this also relies on the USB 3 device detecting which way round the connector is. USB 2 has no such detection, as it's connectors were never rotationally symmetrical. So I guess it's your USB 2 mouse not being able to detect the connection has been inverted.