A bit more digging. Running tcpdump on the server network interface, it seems not all client requests are being replied to:
18:17:19.396648 IP 90.218.229.222.ntp > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:19.397041 IP bt-inner.aerosys.ntp > 90.218.229.222.ntp: NTPv4, Server, length 48
18:17:19.422256 IP 90.218.229.222.ntp > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:19.696881 IP 185.165.160.213.3017 > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:19.697363 IP bt-inner.aerosys.ntp > 185.165.160.213.3017: NTPv4, Server, length 48
18:17:20.397384 IP 90.218.229.222.ntp > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:21.397657 IP 90.218.229.222.ntp > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:23.880008 IP 86.10.94.208.50297 > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:23.880521 IP bt-inner.aerosys.ntp > 86.10.94.208.50297: NTPv4, Server, length 48
18:17:27.385693 IP 2.102.110.196.51730 > bt-inner.aerosys.ntp: NTPv4, Client, length 48
18:17:27.386123 IP bt-inner.aerosys.ntp > 2.102.110.196.51730: NTPv4, Server, length 48
At lest two in the above don’t get a reply, so the plot thickens. Running FreeBSD 12, so kernel variables ?..