--- a/sys/netinet6/in6_ifattach.c
+++ b/sys/netinet6/in6_ifattach.c
@@ -818
 	/* leave from all multicast groups joined */
 	in6_pcbpurgeif0(&ripcbinfo, ifp);
 	in6_pcbpurgeif0(&udbinfo[0], ifp);
+	/* DF-3308: TCP6 pcbs cache im6o_multicast_ifp too */
+	in6_pcbpurgeif0(&tcbinfo[0], ifp);
  	(+ #include <netinet/tcp_var.h> for tcbinfo)