Yes, you're right, it is not common for ISPs to host services on the gateway IP allocated to the customer; however, it is possible, and it is still important that when sending data to the ISP's gateway (for example, diagnostic purposes with ICMP echo), that it actually reaches the ISP's equipment. I feel like the current implementation of bridge mode on the RUT240 exhibits bad practices. I have an RUT950 (also in bridge mode), and it works exactly as expected in these circumstances. Why can't the RUT240 function in the same way?
I did check, and, yes, the IP address and gateway IP assigned to the RUT240's WAN interface (while not in bridge mode) is the same as what is assigned to my equipment when in bridge mode. This is exactly what I would expect to see.