As it turned out, the problem was on the client-side dhcpv6c and was fixed in T2510
Next Rolling release will be without this bug.
vyos@r2-roll:~$ sudo tcpdump -ntvi eth1 port 546 tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes IP6 (flowlabel 0x630bb, hlim 1, next-header UDP (17) payload length: 60) fe80::5054:ff:fe78:6e81.546 > ff02::1:2.547: [bad udp cksum 0xbc21 -> 0x1ea8!] dhcp6 solicit (xid=f47a17 (client-ID hwaddr/time type 1 time 651069356 5254004ba2c4) (IA_NA IAID:1 T1:0 T2:0) (elapsed-time 103) (option-request DNS-server DNS-search-list)) IP6 (flowlabel 0x0d161, hlim 64, next-header UDP (17) payload length: 128) fe80::5054:ff:fe3c:7c1f.547 > fe80::5054:ff:fe78:6e81.546: [bad udp cksum 0x8790 -> 0xfa6f!] dhcp6 advertise (xid=f47a17 (IA_NA IAID:1 T1:0 T2:0 (IA_ADDR 2001:db8::199 pltime:27000 vltime:43200)) (client-ID hwaddr/time type 1 time 651069356 5254004ba2c4) (server-ID hwaddr/time type 1 time 651085104 5254003c7c1f) (DNS-server 2001:db8::ffff 2001:db8::1)) IP6 (flowlabel 0x630bb, hlim 1, next-header UDP (17) payload length: 106) fe80::5054:ff:fe78:6e81.546 > ff02::1:2.547: [bad udp cksum 0xbc4f -> 0x44a5!] dhcp6 request (xid=a568a (client-ID hwaddr/time type 1 time 651069356 5254004ba2c4) (server-ID hwaddr/time type 1 time 651085104 5254003c7c1f) (IA_NA IAID:1 T1:0 T2:0 (IA_ADDR 2001:db8::199 pltime:27000 vltime:43200)) (elapsed-time 0) (option-request DNS-server DNS-search-list)) IP6 (flowlabel 0x0d161, hlim 64, next-header UDP (17) payload length: 128) fe80::5054:ff:fe3c:7c1f.547 > fe80::5054:ff:fe78:6e81.546: [bad udp cksum 0x8790 -> 0x19e7!] dhcp6 reply (xid=a568a (IA_NA IAID:1 T1:0 T2:0 (IA_ADDR 2001:db8::199 pltime:27000 vltime:43200)) (client-ID hwaddr/time type 1 time 651069356 5254004ba2c4) (server-ID hwaddr/time type 1 time 651085104 5254003c7c1f) (DNS-server 2001:db8::ffff 2001:db8::1))