In RouterOS's case, when you reboot the router, it doesn't notify the other devices in LAN that the old prefix should be deprecated, it just reboots. After the reboot of course the router also has no knowledge about the prefix from the previous boot, and will just advertise the new fresh prefix.
On the clients, all prefixes old and new are marked as preferred until the preferred lifetime expires. An upgrade to 7.21rc1 for example, with two reboots (one for the RouterBOARD upgrade) will leave 3 active preferred prefixes on the client devices.
When the prefix changes without the router being rebooted, then the router does correctly advertise the old prefix(es) as deprecated. Maybe it should be modified to save the old prefix information of the interfaces (including remaining lifetime) to storage before the reboot, so that it can still be able to advertise them as deprecated in case the prefixes change after reboot.
Somewhat similar to how the DHCP server remembers active leases at reboot. The DHCP server also periodically saves the leases to storage to deal with unexpected power losses. That could also be done for dynamic IPv6 prefixes being advertised on interfaces.