I just tried this on Ubuntu Jammy and doesn’t appear to be an issue there:
lxc launch images:ubuntu/jammy c1
lxc exec c1 -- apt install acl -y
lxc exec c1 -- groupadd foo
lxc exec c1 -- setfacl -m g:foo:rwx ./foo
lxc exec c1 -- getfacl foo
# file: foo
# owner: root
# group: root
user::rw-
group::r--
group:foo:rwx
mask::rwx
other::r--
lxc stop c1
lxc publish c1
Instance published with fingerprint: e20a9ff0b6e6877d2680ee8798db97a0ab0cd735f72047988ea3a4dfe95a60d5
lxc delete -f c1
lxc launch e20a9ff0b6e6877d2680ee8798db97a0ab0cd735f72047988ea3a4dfe95a60d5 c1
Creating c1
Starting c1
lxc exec c1 -- getfacl foo
# file: foo
# owner: root
# group: root
user::rw-
group::r--
group:foo:rwx
mask::rwx
other::r--
lxc export c1 /home/user/c1.tar.gz
lxc delete -f c1
lxc import /home/user/c1.tar.gz
lxc start c1
lxc exec c1 -- getfacl foo
# file: foo
# owner: root
# group: root
user::rw-
group::r--
group:foo:rwx
mask::rwx
other::r--
What LXD version and host OS is this?