diff --git a/federation/server.go b/federation/server.go
index 27f3c3a1..ee9cd2c4 100644
--- a/federation/server.go
+++ b/federation/server.go
@@ -357,6 +357,57 @@ func (s *Server) MustCreateEvent(t ct.TestLike, room *ServerRoom, ev Event) goma
 	return signedEvent
 }
 
+// Invite someone a room.
+func (s *Server) MustInviteRoom(
+	t ct.TestLike,
+	deployment FederationDeployment,
+	remoteServer spec.ServerName,
+	roomID string,
+	inviterUserID string,
+	inviteeUserID string,
+) {
+	t.Helper()
+	fedClient := s.FederationClient(deployment)
+
+	room := s.rooms[roomID]
+	if room == nil {
+		ct.Fatalf(t, "MustInviteRoom: room %s not found", roomID)
+	}
+	roomVersionImplementation, err := gomatrixserverlib.GetRoomVersion(room.Version)
+
+	rawInviteEvent := s.MustCreateEvent(t, room, Event{
+		Type:     "m.room.member",
+		StateKey: &inviterUserID,
+		Sender:   inviteeUserID,
+		Content: map[string]interface{}{
+			"membership": "invite",
+		},
+	})
+	inviteReq, err := fclient.NewInviteV2Request(rawInviteEvent, []gomatrixserverlib.InviteStrippedState{})
+	if err != nil {
+		t.Fatalf("failed to make invite request: %s", err)
+	}
+	sendInviteResp, err := fedClient.SendInviteV2(
+		context.Background(),
+		spec.ServerName(s.ServerName()),
+		remoteServer,
+		inviteReq,
+	)
+	if err != nil {
+		t.Fatalf("MustInviteRoom: failed to send invite v2: %s", err)
+	}
+
+	inviteEvent, err := roomVersionImplementation.NewEventFromUntrustedJSON(sendInviteResp.Event)
+	if err != nil {
+		t.Fatalf("MustInviteRoom: failed to decode event response: %w", err)
+	}
+
+	room.AddEvent(inviteEvent)
+	s.rooms[roomID] = room
+
+	t.Logf("Server.MustInviteRoom %s invited %s to room ID %s", inviterUserID, inviteeUserID, roomID)
+}
+
 // MustJoinRoom will make the server send a make_join and a send_join to join a room
 // It returns the resultant room.
 func (s *Server) MustJoinRoom(t ct.TestLike, deployment FederationDeployment, remoteServer spec.ServerName, roomID string, userID string, partialState ...bool) *ServerRoom {
diff --git a/tests/federation_rooms_invite_test.go b/tests/federation_rooms_invite_test.go
index 896bf9f7..12ab64ba 100644
--- a/tests/federation_rooms_invite_test.go
+++ b/tests/federation_rooms_invite_test.go
@@ -13,6 +13,7 @@ import (
 	"github.com/tidwall/gjson"
 
 	"github.com/matrix-org/complement/client"
+	"github.com/matrix-org/complement/federation"
 	"github.com/matrix-org/complement/helpers"
 	"github.com/matrix-org/complement/match"
 	"github.com/matrix-org/complement/must"
@@ -99,7 +100,7 @@ func TestFederationRoomsInvite(t *testing.T) {
 			verifyState(t, res, wantFields, wantValues, roomID, alice)
 		})
 
-		t.Run("Remote invited user can join the room when homeserver is already participating in the room", func(t *testing.T) {
+		t.Run("Remote invited user can join the room when homeserver is already participating in the room (e2e)", func(t *testing.T) {
 			t.Parallel()
 			roomID := alice.MustCreateRoom(t, map[string]interface{}{
 				"preset": "private_chat",
@@ -121,7 +122,7 @@ func TestFederationRoomsInvite(t *testing.T) {
 			alice.MustSyncUntil(t, client.SyncReq{}, client.SyncJoinedTo(bob2.UserID, roomID))
 		})
 
-		t.Run("Remote invited user can reject invite when homeserver is already participating in the room", func(t *testing.T) {
+		t.Run("Remote invited user can reject invite when homeserver is already participating in the room (e2e)", func(t *testing.T) {
 			t.Parallel()
 			roomID := alice.MustCreateRoom(t, map[string]interface{}{
 				"preset": "private_chat",
@@ -143,6 +144,44 @@ func TestFederationRoomsInvite(t *testing.T) {
 			alice.MustSyncUntil(t, client.SyncReq{Filter: includeLeaveSyncFilter}, client.SyncLeftFrom(bob2.UserID, roomID))
 		})
 
+		// Engineered condition where we only send `/invite` requests over federation and do
+		// not send the event in a `/send` transaction. Regression tests for
+		// https://github.com/element-hq/synapse/pull/18075
+		t.Run("Remote invited user can join the room when homeserver is already participating in the room (engineered)", func(t *testing.T) {
+			t.Parallel()
+
+			srv := federation.NewServer(t, deployment,
+				federation.HandleKeyRequests(),
+				// bob1 will use this to join the remote room
+				federation.HandleMakeSendJoinRequests(),
+			)
+			cancel := srv.Listen()
+			defer cancel()
+
+			engineeredAliceUserId := srv.UserID("alice")
+
+			roomVersion := alice.GetDefaultRoomVersion(t)
+			initalEvents := federation.InitialRoomEvents(roomVersion, engineeredAliceUserId)
+			serverRoom := srv.MustMakeRoom(t, roomVersion, initalEvents)
+			roomID := serverRoom.RoomID
+
+			// bob1 is invited and can join the room (hs2 is now participating of the room)
+			//
+			// alice.MustInviteRoom(t, roomID, bob.UserID)
+			srv.MustInviteRoom(t, deployment, "hs1", roomID, engineeredAliceUserId, bob.UserID)
+			bob.MustSyncUntil(t, client.SyncReq{}, client.SyncInvitedTo(bob.UserID, roomID))
+			bob.MustJoinRoom(t, roomID, []string{srv.ServerName()})
+			// Make sure alice can see bob in the room
+			// alice.MustSyncUntil(t, client.SyncReq{}, client.SyncJoinedTo(bob.UserID, roomID))
+
+			// bob2 is invited and can also join the room
+			// alice.MustInviteRoom(t, roomID, bob2.UserID)
+			// bob2.MustSyncUntil(t, client.SyncReq{}, client.SyncInvitedTo(bob2.UserID, roomID))
+			// bob2.MustJoinRoom(t, roomID, []string{srv.ServerName()})
+			// // Make sure alice can see bob2 in the room
+			// alice.MustSyncUntil(t, client.SyncReq{}, client.SyncJoinedTo(bob2.UserID, roomID))
+		})
+
 		t.Run("Invited user has 'is_direct' flag in prev_content after joining", func(t *testing.T) {
 			roomID := alice.MustCreateRoom(t, map[string]interface{}{
 				"preset": "private_chat",