Ilya Yusupov from Check Point was immensely helpful in tracking this down, installing the hotfix for sk134054 (Rare failure in the Identity Sharing network registration may potentially result in incorrect policy...) resolved the problem:
Gateway was running R80.10 with JHF 121 and sk134253 (Check Point response to SegmentSmack & FragmentSmack) with Identity Awareness blade inactive:
[Expert@fwcp1:0]# enabled_blades
fw vpn urlf av appi ips anti_bot mon vpn
The network security policy exclusively had the firewall blade active:
The problem appears to occur when a policy rule references identity awareness data and there is either a failure obtaining identities (eg the original SK where identity sharing was unavailable) or when the policy includes a rule which had been structured for imminent activation of the Identity Awareness blade: