The point is that for the route via WAN1 to stay active, it is sufficient that the gateway on the Huawei modem is pingable. Hence even if you disconnect the fibre uplink from the Huawei, Mikrotik keeps sending packets via WAN1.
Instead, you need to make the route via WAN1 check that the whole uplink path is transparent all the way to the internet - see this ultimate post by @Chupaka for the details.