diff --git a/internal/caching/cache_lazy_load_members.go b/internal/caching/cache_lazy_load_members.go
index 0d7009c9..390334da 100644
--- a/internal/caching/cache_lazy_load_members.go
+++ b/internal/caching/cache_lazy_load_members.go
@@ -14,6 +14,7 @@ type lazyLoadingCacheKey struct {
 type LazyLoadCache interface {
 	StoreLazyLoadedUser(device *userapi.Device, roomID, userID, eventID string)
 	IsLazyLoadedUserCached(device *userapi.Device, roomID, userID string) (string, bool)
+	InvalidateLazyLoadedUser(device *userapi.Device, roomID, userID string)
 }
 
 func (c Caches) StoreLazyLoadedUser(device *userapi.Device, roomID, userID, eventID string) {
@@ -33,3 +34,12 @@ func (c Caches) IsLazyLoadedUserCached(device *userapi.Device, roomID, userID st
 		TargetUserID: userID,
 	})
 }
+
+func (c Caches) InvalidateLazyLoadedUser(device *userapi.Device, roomID, userID string) {
+	c.LazyLoading.Unset(lazyLoadingCacheKey{
+		UserID:       device.UserID,
+		DeviceID:     device.ID,
+		RoomID:       roomID,
+		TargetUserID: userID,
+	})
+}
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index 990ca55b..b4c9a542 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -20,6 +20,10 @@ import (
 	"net/http"
 	"sort"
 
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
+
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/internal/caching"
 	"github.com/matrix-org/dendrite/roomserver/api"
@@ -28,9 +32,6 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/sync"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
-	"github.com/matrix-org/gomatrixserverlib"
-	"github.com/matrix-org/util"
-	"github.com/sirupsen/logrus"
 )
 
 type messagesReq struct {
@@ -262,7 +263,7 @@ func (m *messagesResp) applyLazyLoadMembers(
 		}
 	}
 	for _, evt := range membershipToUser {
-		m.State = append(m.State, gomatrixserverlib.HeaderedToClientEvent(evt, gomatrixserverlib.FormatSync))
+		m.State = append(m.State, gomatrixserverlib.HeaderedToClientEvent(evt, gomatrixserverlib.FormatAll))
 	}
 }
 
diff --git a/syncapi/streams/stream_pdu.go b/syncapi/streams/stream_pdu.go
index 1003208f..1ad3adc4 100644
--- a/syncapi/streams/stream_pdu.go
+++ b/syncapi/streams/stream_pdu.go
@@ -12,9 +12,12 @@ import (
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
+
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/tidwall/gjson"
 	"go.uber.org/atomic"
+
+	"github.com/matrix-org/dendrite/syncapi/notifier"
 )
 
 // The max number of per-room goroutines to have running.
@@ -34,6 +37,7 @@ type PDUStreamProvider struct {
 	// userID+deviceID -> lazy loading cache
 	lazyLoadCache caching.LazyLoadCache
 	rsAPI         roomserverAPI.SyncRoomserverAPI
+	notifier      *notifier.Notifier
 }
 
 func (p *PDUStreamProvider) worker() {
@@ -100,6 +104,15 @@ func (p *PDUStreamProvider) CompleteSync(
 		req.Log.WithError(err).Error("unable to update event filter with ignored users")
 	}
 
+	// Invalidate the lazyLoadCache, otherwise we end up with missing displaynames/avatars
+	// TODO: This might be inefficient, when joined to many and/or large rooms.
+	for _, roomID := range joinedRoomIDs {
+		joinedUsers := p.notifier.JoinedUsers(roomID)
+		for _, sharedUser := range joinedUsers {
+			p.lazyLoadCache.InvalidateLazyLoadedUser(req.Device, roomID, sharedUser)
+		}
+	}
+
 	// Build up a /sync response. Add joined rooms.
 	var reqMutex sync.Mutex
 	var reqWaitGroup sync.WaitGroup
diff --git a/syncapi/streams/streams.go b/syncapi/streams/streams.go
index 1ca4ee8c..dbc053bd 100644
--- a/syncapi/streams/streams.go
+++ b/syncapi/streams/streams.go
@@ -34,6 +34,7 @@ func NewSyncStreamProviders(
 			StreamProvider: StreamProvider{DB: d},
 			lazyLoadCache:  lazyLoadCache,
 			rsAPI:          rsAPI,
+			notifier:       notifier,
 		},
 		TypingStreamProvider: &TypingStreamProvider{
 			StreamProvider: StreamProvider{DB: d},