You don't mention which ePO patch is installed, but it looks a lot like an issue which is resolved in ePO 4.6 Patch 3.
Per the patch 3 readme:
77. Issue – Agents occasionally return incorrect ipV6subnet information that suggests all IPs in the network are in the subnet. (Reference: 699813)
Resolution – Invalid subnet records are removed upon installation of ePolicy Orchestrator 4.6.3. In addition, RSD ignores invalid subnet values from agents.