If you don't want that the router announces itself as the gateway on an interface then you should create a /ipv6 nd for that interface with ra-lifetime set to none (greyed out in WinBox).
If you don't want the router to be the IPv6 gateway on any interface at all, then instead of creating the individual entries, you can also just modify the default /ipv6 nd entry with the all interface and set ra-lifetime=none on that.
This is probably the change in 7.23 that causes the behavior you observed:
Previous versions apparently didn't send out RA when there was no prefix (advertise turned off in /ipv6 address entries). Version 7.23 fixed that and also sends router advertisement if you have value set for DNS or MTU im the ND entry.