It is the way it works. Essentially, all static maps need to be outside the normal range because of the way the underlying software is written. It also forces you to keep your DHCP range clean. But mainly it is because trying to make in scope reservations is not clean... At least I could not find a clean way to do it.
Hmmmm...(scratching head)...I have a series of existing entries which are in the DHCP range. I must have put them in years ago under who knows what version of m0n0nwall. If I understand you correctly, the persistent (static?) DHCP addresses are still DHCP but outside of the range defined in the DHCP server settings?
As a sanity check, I will release the DHCP lease and power down the server in question, and then try again.