This is not a bug, it's intended VIP and IP-Pool behavior from 7.0.13
onwards.
https://community.fortinet.com/t5/FortiGate/Technical-Tip-ARP-reply-setting-in-Virtual-IP-IP-Pool/ta-p/192527
Before FortiOS 6.4.9 / 7.0.1 all IP addresses in the IP pool ...
What is the exact workflow you used when deleting the per-device mapping
and installing the policy package? When does the per-device mapping
reappear? After running the install wizard?