Currently you can only see rooms you are either joined to or which have History Visibility set to Anyone. Without one of those you are unable to fetch details about the room such as name/avatar/topic/members which is needed for the UI. The solution for this in the long term is matrix-org/matrix-spec-proposals#3173 which will allow you to fetch the necessary details for any room if you are able to join it without requiring an invite.
For the private space/rooms edge of this is matrix-org/matrix-spec-proposals#3083 which will allow members of a space access to a room which would also grant them access to its metadata to be able to see it prior to joining it.
Another situation which might cause this issue is if you are joined to some rooms but they are contained within a subspace you are not joined to; unsure of what the final solution will be here but once /peek
MSC is stable that might provide a solution - https://github.com/vector-im/ele