I just upgraded version to OPNsense 21.1.9_1-amd64 yesterday. This morning I set a DHCP IP Address Lease to static so a device on my network would always have the same IP Address. I did click "Apply" after saving the changes. It is a new WiFi device I was just configuring for my home automation system. I unplugged the device to move it to it's final location. A little while later my daughter came home and her iPhone was assigned the same IP Address I had just reserved for the home automation device. The attached file is a couple of screenshots showing the leases, and the reservation.