Repro steps (I hope):

• A room exists like #seaglass:matrix.org
• A user joins it from a virginal HS (e.g. @neil:sandbox.modular.im)
• They do not yet backpaginate any history for that room over federation for whatever reason
• Another user joins the room from the same HS (e.g. @matthew:sandbox.modular.im)
• The /sync as the new user joins it does not include a backwards pagination token at all (we think, assuming this is not a js-sdk bug).
• Therefore the new user can never paginate backwards in the room to pull in old history.