A unicast frame for the bridge is handed to the stack with rcvif set
to the bridge, but it kept the FIB of the member it arrived on. With
the bridge in another FIB, replies were looked up in the wrong routing
table and dropped.

Set the FIB of the bridge on such frames. If member_ifaddrs is
enabled and the bridge inherited its MAC address from a member, keep
the old behaviour, since the frame may be for an address configured on
that member.