https://github.com/nextcloud/mail/pull/4170 was incomplete:

forwarding a message from a threadEnvelope's forward menu action has the
same problem

Signed-off-by: Cyrille Bollu <cyr.debian@bollu.be>

src/components/ThreadEnvelope.vue

@@ -346,7 +346,7 @@ export default {
 					filter: this.$route.params.filter ? this.$route.params.filter : undefined,
 				},
 				query: {
-					messageId: this.$route.params.threadId,
+					messageId: this.envelope.databaseId,
 				},
 			})
 		},