diff --git a/federationapi/consumers/roomserver.go b/federationapi/consumers/roomserver.go
index 80317ee69..32e0ca2b8 100644
--- a/federationapi/consumers/roomserver.go
+++ b/federationapi/consumers/roomserver.go
@@ -164,7 +164,7 @@ func (s *OutputRoomEventConsumer) processMessage(ore api.OutputNewRoomEvent) err
 		addsStateEvents = append(addsStateEvents, eventsRes.Events...)
 	}
 
-	addsJoinedHosts, err := joinedHostsFromEvents(gomatrixserverlib.UnwrapEventHeaders(addsStateEvents))
+	addsJoinedHosts, err := JoinedHostsFromEvents(gomatrixserverlib.UnwrapEventHeaders(addsStateEvents))
 	if err != nil {
 		return err
 	}
@@ -238,7 +238,7 @@ func (s *OutputRoomEventConsumer) joinedHostsAtEvent(
 		return nil, err
 	}
 
-	combinedAddsJoinedHosts, err := joinedHostsFromEvents(combinedAddsEvents)
+	combinedAddsJoinedHosts, err := JoinedHostsFromEvents(combinedAddsEvents)
 	if err != nil {
 		return nil, err
 	}
@@ -284,10 +284,10 @@ func (s *OutputRoomEventConsumer) joinedHostsAtEvent(
 	return result, nil
 }
 
-// joinedHostsFromEvents turns a list of state events into a list of joined hosts.
+// JoinedHostsFromEvents turns a list of state events into a list of joined hosts.
 // This errors if one of the events was invalid.
 // It should be impossible for an invalid event to get this far in the pipeline.
-func joinedHostsFromEvents(evs []*gomatrixserverlib.Event) ([]types.JoinedHost, error) {
+func JoinedHostsFromEvents(evs []*gomatrixserverlib.Event) ([]types.JoinedHost, error) {
 	var joinedHosts []types.JoinedHost
 	for _, ev := range evs {
 		if ev.Type() != "m.room.member" || ev.StateKey() == nil {
diff --git a/federationapi/internal/perform.go b/federationapi/internal/perform.go
index 577cb70e0..24a4d2f10 100644
--- a/federationapi/internal/perform.go
+++ b/federationapi/internal/perform.go
@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/matrix-org/dendrite/federationapi/api"
+	"github.com/matrix-org/dendrite/federationapi/consumers"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/version"
 	"github.com/matrix-org/gomatrix"
@@ -235,6 +236,20 @@ func (r *FederationInternalAPI) performJoinUsingServer(
 		return fmt.Errorf("respSendJoin.Check: %w", err)
 	}
 
+	// We need to immediately update our list of joined hosts for this room now as we are technically
+	// joined. We must do this synchronously: we cannot rely on the roomserver output events as they
+	// will happen asyncly. If we don't update this table, you can end up with bad failure modes like
+	// joining a room, waiting for 200 OK then changing device keys and have those keys not be sent
+	// to other servers (this was a cause of a flakey sytest "Local device key changes get to remote servers")
+	// The events are trusted now as we performed auth checks above.
+	joinedHosts, err := consumers.JoinedHostsFromEvents(respState.StateEvents.TrustedEvents(respMakeJoin.RoomVersion, false))
+	if err != nil {
+		return fmt.Errorf("JoinedHostsFromEvents: failed to get joined hosts: %s", err)
+	}
+	if _, err = r.db.UpdateRoom(context.Background(), roomID, "", event.EventID(), joinedHosts, nil); err != nil {
+		return fmt.Errorf("UpdatedRoom: failed to update room with joined hosts: %s", err)
+	}
+
 	// If we successfully performed a send_join above then the other
 	// server now thinks we're a part of the room. Send the newly
 	// returned state to the roomserver to update our local view.