Neighbor discovery for IPv6 may be blocked by Checkpoint anti-spoofing.There may be instances in which neighbor solicitation packets for IPv6 may not be replied to by the firewall.
After populating interfaces with IPv6 alias addresses within the Checkpoint GUI you may run a fw ctl zdebug drop and see the the neighbor solicitation packets be dropped due to anti-spoofing.
In order to resolve this, you will need to adjust your anti-spoofing settings on the interfaces in which the problem occurs.