First message missing on DMs
I've observed at least one case where I sent a DM to a synapse user from element and they did not receive the first message. Looking at their database afterwards, the event was missing completely. @tulir:maunium.net
stated that this is a common behavior with conduit servers.
I haven't been able to reproduce it so far with local users, or with two federating grapevine instances. It may be specific to grapevine/synapse interaction.