I will try to get time to test with tcpdump and try to understand the capture with wireshark.
But after looking for more information I think it may be a problem specific to my device (Turris Omnia) and its DTS / DTB:
- https://github.com/openwrt/openwrt/issues/12366
- https://forum.turris.cz/t/multicast-igmp-reports-missing/18192/14
And the symptoms seem to fit with the ones I had using IGM Snooping with DHCPv6 (IPv6 fail on wired network and work on wireless network):