Contacted ISP and half hour later the problem is solved. Don't know what issue was.
Thank you all for yours support.
I think I've got the same issue now.
My clients get multiple default routes for IPv6 coming apparently from my APs (which are mainly just CAPsMAN APs and not really routers).
advertise flag under /ipv6 address is not enabled on those routers though. Maybe that is a bug in 7.23? What can I do to stop those devices to send RA advertisements then?
As my issue involved ISP, I don't know exactly the solution.
What helped me, was to disable IPv6 on secondary Mikrotik routers.
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.