MSC1772 says (emphasis mine):

Rooms are sorted based on a lexicographic ordering of the Unicode codepoints of the characters in order values. Rooms with no order come last, in ascending numeric order of the origin_server_ts of their m.room.create events, or ascending lexicographic order of their room_ids in case of equal origin_server_ts.

However, Element Web currently puts rooms with no order at the end of the list in the space view, rather than at the beginning.I think it also doesn't sort the orderless rooms by origin_server_ts as the MSC says (related: #18763). This is only about the space view, as the room list in the sidebar doesn't do any space-based sorting yet.

I'm not completely sure if this is a synapse or element web bug, but element android seems to sort the rooms correctly (at least the order-based ones, not sure about unordered ones based on timestamp), so I'm guessing it's web.

Steps to reproduce

1. Create a space
2. Add m.space.child state events where some of them have the order field and some don't
3. Observe the rooms with order at the bottom of the list instead of the top