Client is not properly backfilling room history with federation

[romanicandrea]

We’re using Synapse/1.105.1 on the server side and matrix-js-sdk in our react native app. We have two servers, server A and server B. The room is created on server A, and users are invited, one from server A and one from server B. The user A joins the room and sends the message. Afterwards, the user B joins, but the message or anything else from before the /join is not federated, the room is there, everything seems okay, but the history is missing. Only after we send something else from server A, for example another message, this triggers or rather pulls the federation of the history as well. History visibility is set to shared, everything works as intended afterwards.

Calling API manually (from Postman or calling the api directly outside of the client for example) triggers the federation properly: calling /sync and afterwards /messages works as intended, so this is most likely not a Synapse issue. However, we cannot make the client backfill the history.

Steps we do:

1. Receive membership event
2. Join the room
3. Get room live timeline
4. The timeline is empty and stays empty
5. Another message is sent from the other server
6. The timeline is backfilled

We also tried paginating backwards, we get state events prior to the first message, but not the message itself.

Is there a solution for this, can we somehow make sure that the history is available when the user joins the room? I found similar issues but nothing exactly the same nor the explanation how to force syncing the room.