I would open a ticket if possible with tac. They may take the packet captures like tcpdump and fw monitor and perhaps a kernel debug might also be required in few cases. Hence better if TAC can take a look.
In Check point, the NAT64 gateway creates a mapping between the IPv6 and the IPv4 addresses, which may be manually configured or determined automatically. The original destination IPv6 address is linked with the translated destination IPv4 address so it must be unique. What that means is that the IPv6 and IPv4 address must be different in each rule.
For example, if you are using two different IPv6 address in the Original Destination but the Translated Destination has the same IPv4 address in both rules then it might not work
Since we cant see your configuration , please contact TAC for deeper analysis