I assumed that an empty bridge without interfaces would not communicate with network devices in other bridges.
But I see something different.
On Mikrotik hAP ac2 with default configuration with address 192.168.88.1 I do that:
/interface bridge add name=bridge2
/ip address add address=192.168.17.1/24 interface=bridge2 network=192.168.17.0
And after that router becomes accessible at the address 192.168.17.1 from devices on the subnet 192.168.88.0/24
Why is this happening?
Just routing.
Devices probably have this router as default gateway, so they will seek other subnets via 192.168.88.1
All interfaces are routable if forwarding is not dropped by the firewall.
And using IP address which is router’s own address on that “non-populated” bridge is separate issue any way. Because when router processes packets, it first checks if the packet is targeting any of its own addresses (and performs appropriate action, DST-NAT is one possibility, servicing requests natively is another possibility). Only after that does the routing (in which case packet would hit the other bridge where it would normally be handled by L2 logic).