I've found that encap6_input() is not registered for IPPROTO_ETHERIP and that bridge(4) and gif(4) will not work over an IPv6-IPv6 tunnel. So, I've tried to add below code and found that etherip over ipv6 will work after this fix.
A commit in branch main references this bug:
Author: Hiroki Sato <hrs@FreeBSD.org>
AuthorDate: 2021-08-27 08:14:35 +0000
Commit: Hiroki Sato <hrs@FreeBSD.org>
CommitDate: 2021-08-27 08:14:35 +0000
inet6(4): add a missing IPPROTO_ETHERIP entry
bridge(4) + gif(4) did not work when the outer protocol was IPv6.
Submitted by: Masahiro Kozuka
MFC after: 3 days
sys/netinet6/in6_proto.c | 9 +++++++++
1 file changed, 9 insertions(+)
This will be merged into stable branches.