Below is part of the diff between a working config file and that which is written by keama:
--- /etc/kea/kea-dhcp4.conf	2025-04-09 23:41:24.792270901 +1000
+++ /root/kea-dhcp4.conf	2025-04-09 23:39:14.501232144 +1000
@@ -93,6 +93,7 @@
     "host-reservation-identifiers": [
       "hw-address"
     ],
+    "reservation-mode": "global",
     "reservations": [
       {
         "hostname": "xev",

I think it should generate a config file that just works.  I deleted that line
and it worked.

Dear submitter,

as the package isc-dhcp has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1143544

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

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Thorsten Alteholz (the ftpmaster behind the curtain)