If the client sends that as its hostname.. Then ok - but dhcp leases shouldn't be showing a fqdn.. It would only be showing the hostname.
If you want client amazon-random# to show up as alexa-name in your dhcp lease. The correct solution is to either have that specific client send that hostname to the dhcpd, which I don't think you can do on alexa. Or tell the dhcp server to use hostname xyz in the host name when you set a reservation.
If your setting reservations for your clients, and register that in dhcp settings - then all your dns is taken care of.