This bug existed prior to #1700 and was caused by not clearing the buffers that were already shared with a peer that left and opened a project using the same connection. When such peer would re-join the project and open a buffer that it had opened previously, the host assumed the peer had already seen that buffer and wouldn't bother sending it again.
Zed Server
This crate is what we run at https://collab.zed.dev.
It contains our back-end logic for collaboration, to which we connect from the Zed client via a websocket after authenticating via https://zed.dev, which is a separate repo running on Vercel.