Looks like the requests are there: two MAC addresses, since I now have two test containers (with the same problem)
matteo@matteo-laptop:~$ sudo tcpdump -ni lxdbr0
[sudo] password for matteo:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lxdbr0, link-type EN10MB (Ethernet), capture size 262144 bytes
19:18:15.807692 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:c6:e1:65, length 300
19:18:24.427846 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:c6:e1:65, length 300
19:18:25.927684 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:6f:3b:f6, length 300
19:18:37.803704 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:c6:e1:65, length 300
19:18:40.309507 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:6f:3b:f6, length 300
19:18:40.344268 IP6 fe80::c846:67ff:fe11:7f4.5353 > ff02::fb.5353: 0 [2q] PTR (QM)? _ipp._tcp.local. PTR (QM)? _ipps._tcp.local. (45)
19:18:52.737047 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:6f:3b:f6, length 300
19:18:56.607963 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:c6:e1:65, length 300
19:19:06.198260 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:c6:e1:65, length 300
19:19:06.412253 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:16:3e:6f:3b:f6, length 300
^C
10 packets captured
10 packets received by filter
0 packets dropped by kernel