ip6_input_if() and ip6_output() belongs to different paths. Nothing
wrong to reload `ip6_mforwarding' while the interface was changed in
ip6_output().

Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
diff -u -p -r1.144 in6_proto.c
--- sys/netinet6/in6_proto.c	24 Jul 2025 22:57:24 -0000	1.144
+++ sys/netinet6/in6_proto.c	25 Jul 2025 09:09:23 -0000
@@ -349,7 +349,7 @@ const struct domain inet6domain = {
  * Internet configuration info
  */
 int	ip6_forwarding = 0;	/* [a] no forwarding unless sysctl to enable */
-int	ip6_mforwarding = 0;	/* no multicast forwarding unless ... */
+int	ip6_mforwarding = 0;	/* [a] no multicast forwarding unless ... */
 int	ip6_multipath = 0;	/* [a] no using multipath routes unless ... */
 int	ip6_sendredirects = 1;	/* [a] */
 int	ip6_defhlim = IPV6_DEFHLIM;			/* [a] */
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
diff -u -p -r1.289 ip6_input.c
--- sys/netinet6/ip6_input.c	24 Jul 2025 22:57:24 -0000	1.289
+++ sys/netinet6/ip6_input.c	25 Jul 2025 09:09:23 -0000
@@ -477,7 +477,8 @@ ip6_input_if(struct mbuf **mp, int *offp
 			ours = 1;
 
 #ifdef MROUTING
-		if (ip6_mforwarding && ip6_mrouter[ifp->if_rdomain]) {
+		if (atomic_load_int(&ip6_mforwarding) &&
+		    ip6_mrouter[ifp->if_rdomain]) {
 			int error;
 
 			nxt = ip6_hbhchcheck(&m, offp, &ours, flags);
@@ -1455,10 +1456,10 @@ const struct sysctl_bounded_args ipv6ctl
 	{ IPV6CTL_DEFMCASTHLIM, &ip6_defmcasthlim, 0, 255 },
 	{ IPV6CTL_USE_DEPRECATED, &ip6_use_deprecated, 0, 1 },
 	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
+	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
 };
 
 const struct sysctl_bounded_args ipv6ctl_vars[] = {
-	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
 	{ IPV6CTL_MCAST_PMTU, &ip6_mcast_pmtu, 0, 1 },
 	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
 	{ IPV6CTL_MAXDYNROUTES, &ip6_maxdynroutes, -1, 5 * 4096 },
@@ -1575,6 +1576,7 @@ ip6_sysctl(int *name, u_int namelen, voi
 	case IPV6CTL_DEFMCASTHLIM:
 	case IPV6CTL_USE_DEPRECATED:
 	case IPV6CTL_MAXFRAGS:
+	case IPV6CTL_MFORWARDING:
 		return (sysctl_bounded_arr(
 		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));
Index: sys/netinet6/ip6_output.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_output.c,v
diff -u -p -r1.302 ip6_output.c
--- sys/netinet6/ip6_output.c	24 Jul 2025 19:49:08 -0000	1.302
+++ sys/netinet6/ip6_output.c	25 Jul 2025 09:09:23 -0000
@@ -529,7 +529,8 @@ reroute:
 			 * above, will be forwarded by the ip6_input() routine,
 			 * if necessary.
 			 */
-			if (ip6_mforwarding && ip6_mrouter[ifp->if_rdomain] &&
+			if (atomic_load_int(&ip6_mforwarding) &&
+			    ip6_mrouter[ifp->if_rdomain] &&
 			    (flags & IPV6_FORWARDING) == 0) {
 				if (ip6_mforward(ip6, ifp, m, flags) != 0) {
 					m_freem(m);