I joined a room and was immediately kicked. After some time, I went back to click the "Rejoin" button in the banner at the top of the room and was met with "No known servers".
My attempt to join was done with /join #alias:server.com.
Did you attempt to rejoin manually afterwards with any success? Could it be that synapse is being odd rather than the rejoin button not working?
I manually issued the same /join command and got through. From looking at logs, it appears as though Riot wasn't sending any kind of server information, so synapse had zero chance of finding a resident homeserver. In theory Riot could send the alias instead of the room ID and possibly pick a domain to help if it wanted.
Magically fixed with https://github.com/matrix-org/matrix-react-sdk/pull/2250 (according to my testing)