OSPFv3 LS updates of the default route are not accepted by the Firewall for Stub/TSA areas
According to the OSPFv3 RFC, prefixes need to be padded to an even number of 4 byte words. Check Point gateways do this even for the ::/0 prefix, meaning that they pad 4 bytes of 0's.
Because of how routed deals with ::/0, it was assuming that the minimum size of an inter area prefix LSA is 4 bytes bigger than it should have been. Therefore when it was getting the default route it was ignoring it and recording that as a "Bad Inter Area Prefix LSA size" error.