Sorry for the delay. I cannot reproduce. For the last example, what was the conflict error?
So my understanding is you have an existing (provisioned) host with a working reservation. Then you click on Build button in the Foreman UI and it fails with a conflict?
If a newly provisioned host is provisioned with address and nothing change it (e.g. operator or facts coming out from puppet/ansible/rhsm), no DHCP will be done. There is no need to update reservation, if you are booting from network Foreman will just change PXE bootloader configuration.