Have any of the other users been active on your Discourse since you created the channel? Or logged out and logged back in again?
I just tried to recreate this and yeah, I was only seeing one member in the channel right after creation. Every time I logged in as each of the other users that had access to the category, the member count would go up. Those users were able to see the full channel history so that didn’t start only from when they logged in.
So it looks like it’s a UX/UI bug - it would definitely be less confusing if that members list was populated immediately with whoever has access!