this could be considered a minor issue BUT the this affects synchronization : new messsages on hidden accounts are missing and no notifications occurs.
mailspring does not save the current display settings until you actualy quit it.
accounts list in the left panel only displays the accounts that were displayed last time you quitted the app for real. shutting down computer with mailspring running in background is not enough.
So select “all accounts”, quit mailspring, restart mailspring, done