Currently we have Teams defaulted to show you the first (up to five) channels ever created. If you want to add more channels to your left rail view, just click the ellipses next to the channel name and select “Favorite”. When you favorite the channel, it will then be “pinned” to your left rail under that team name.
Try that and let me know if that meets your needs!
This is still not adressed, wish the dev team could actually reply and say this will be taken care of soon.