Ok, I think I found the issue (not exactly witch was), and get the reservations working without restrictions, even reusing an address from an older device.
When changing from ISC to Kea, I disabled ISC DHCP, but didn't clean the reservation lists, for a situation that I couldn't make kea work in that day, and rollback in an instant.
So after remembering that, I cleaned that list. (Didn't think that could impact, since it was disabled).
But, I also had the option, "Register ISC DHCP4 Leases", in Unbound > General - enabled.
So I cleared the ISC list, and uncheck that unbound option. Restarted, and it worked.
I can't tell if it was both or if I only needed to act in one of them, since I made both changes at the same time, without testing between them.
Thanks @Monviech (Cedrik) for your time in trying to help.
When changing from ISC to Kea, I disabled ISC DHCP, but didn't clean the reservation lists, for a situation that I couldn't make kea work in that day, and rollback in an instant.
So after remembering that, I cleaned that list. (Didn't think that could impact, since it was disabled).
But, I also had the option, "Register ISC DHCP4 Leases", in Unbound > General - enabled.
So I cleared the ISC list, and uncheck that unbound option. Restarted, and it worked.
I can't tell if it was both or if I only needed to act in one of them, since I made both changes at the same time, without testing between them.
Thanks @Monviech (Cedrik) for your time in trying to help.