[trigger]  sendmsg returned -1 (errno=22: Invalid argument)
[after]    sticky IPV6_PKTINFO after buggy sendmsg:
  read-back                  ifindex=0 addr=0000:0000:0000:0000:0000:0000:0000:0000

VERDICT: BUG    - sticky IPV6_PKTINFO was CLEARED (state corruption, DF-0744 reproduced).