diff --git a/tests/federation_room_join_test.go b/tests/federation_room_join_test.go
index 03b12ee0..544bbef2 100644
--- a/tests/federation_room_join_test.go
+++ b/tests/federation_room_join_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/url"
 	"strings"
@@ -574,6 +575,131 @@ func TestSendJoinPartialStateResponse(t *testing.T) {
 	must.HaveInOrder(t, sendJoinResp.ServersInRoom, []string{"hs1"})
 }
 
+// This test verifies that events sent into a room between a /make_join and
+// /send_join are not lost to the joining server. When an event is created
+// during the join handshake, the join event's prev_events (set at make_join
+// time) won't reference it, creating two forward extremities. The server
+// handling the join should ensure the joining server can discover the missed
+// event, for example by sending a follow-up event that references both
+// extremities, prompting the joining server to backfill.
+//
+// See https://github.com/element-hq/synapse/pull/19390
+func TestEventBetweenMakeJoinAndSendJoinIsNotLost(t *testing.T) {
+	deployment := complement.Deploy(t, 1)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Register(t, "hs1", helpers.RegistrationOpts{})
+
+	// We track the message event ID sent between make_join and send_join.
+	// After send_join, we wait for hs1 to send us either:
+	//  - the message event itself, or
+	//  - any event whose prev_events reference the message (e.g. a dummy event)
+	var messageEventID string
+	messageDiscoverableWaiter := helpers.NewWaiter()
+
+	srv := federation.NewServer(t, deployment,
+		federation.HandleKeyRequests(),
+	)
+	srv.UnexpectedRequestsAreErrors = false
+
+	// Custom /send handler: the Complement server won't be in the room until
+	// send_join completes, so we can't use HandleTransactionRequests (which
+	// requires the room in srv.rooms). Instead we parse the raw transaction.
+	srv.Mux().Handle("/_matrix/federation/v1/send/{transactionID}", http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		body, _ := io.ReadAll(req.Body)
+		txn := gjson.ParseBytes(body)
+		txn.Get("pdus").ForEach(func(_, pdu gjson.Result) bool {
+			eventID := pdu.Get("event_id").String()
+			eventType := pdu.Get("type").String()
+			t.Logf("Received PDU via /send: type=%s id=%s", eventType, eventID)
+
+			if messageEventID == "" {
+				return true
+			}
+
+			// Check if this IS the message event (server pushed it directly).
+			if eventID == messageEventID {
+				messageDiscoverableWaiter.Finish()
+				return true
+			}
+
+			// Check if this event's prev_events reference the message
+			// (e.g. a dummy event tying the forward extremities together).
+			pdu.Get("prev_events").ForEach(func(_, prevEvent gjson.Result) bool {
+				if prevEvent.String() == messageEventID {
+					messageDiscoverableWaiter.Finish()
+					return false
+				}
+				return true
+			})
+
+			return true
+		})
+		w.WriteHeader(200)
+		w.Write([]byte(`{"pdus":{}}`))
+	})).Methods("PUT")
+
+	cancel := srv.Listen()
+	defer cancel()
+
+	// Alice creates a room on hs1.
+	roomID := alice.MustCreateRoom(t, map[string]interface{}{
+		"preset": "public_chat",
+	})
+
+	charlie := srv.UserID("charlie")
+	origin := srv.ServerName()
+	fedClient := srv.FederationClient(deployment)
+
+	// Step 1: make_join, hs1 returns a join event template whose prev_events
+	// reflect the current room DAG tips.
+	makeJoinResp, err := fedClient.MakeJoin(
+		context.Background(), origin,
+		deployment.GetFullyQualifiedHomeserverName(t, "hs1"),
+		roomID, charlie,
+	)
+	must.NotError(t, "MakeJoin", err)
+
+	// Step 2: Alice sends a message on hs1. This advances the DAG past the
+	// point captured by make_join's prev_events. The Complement server is not
+	// yet in the room, so it won't receive this event via normal federation.
+	messageEventID = alice.SendEventSynced(t, roomID, b.Event{
+		Type: "m.room.message",
+		Content: map[string]interface{}{
+			"msgtype": "m.text",
+			"body":    "Message sent between make_join and send_join",
+		},
+	})
+	t.Logf("Alice sent message %s between make_join and send_join", messageEventID)
+
+	// Step 3: Build and sign the join event, then send_join.
+	// The join event's prev_events are from step 1 (before the message),
+	// so persisting it on hs1 creates two forward extremities: the message
+	// and the join.
+	verImpl, err := gomatrixserverlib.GetRoomVersion(makeJoinResp.RoomVersion)
+	must.NotError(t, "GetRoomVersion", err)
+	eb := verImpl.NewEventBuilderFromProtoEvent(&makeJoinResp.JoinEvent)
+	joinEvent, err := eb.Build(time.Now(), srv.ServerName(), srv.KeyID, srv.Priv)
+	must.NotError(t, "Build join event", err)
+
+	_, err = fedClient.SendJoin(
+		context.Background(), origin,
+		deployment.GetFullyQualifiedHomeserverName(t, "hs1"),
+		joinEvent,
+	)
+	must.NotError(t, "SendJoin", err)
+
+	// Step 4: hs1 should make the missed message discoverable to the joining
+	// server. We accept either receiving the message event directly, or
+	// receiving any event whose prev_events reference it (allowing the
+	// joining server to backfill).
+	messageDiscoverableWaiter.Waitf(t, 5*time.Second,
+		"Timed out waiting for message event %s to become discoverable — "+
+			"the event sent between make_join and send_join was lost to the "+
+			"joining server", messageEventID,
+	)
+}
+
 // given an event JSON, return the type and state_key, joined with a "|"
 func typeAndStateKeyForEvent(result gjson.Result) string {
 	return strings.Join([]string{result.Map()["type"].Str, result.Map()["state_key"].Str}, "|")