There is no default keybinding related to playback bound to spacebar. Loop Song also has nothing to do with pattern looping or not, it determines whether the song starts playing from the beginning when it reached the end.

There is a multitude of key bindings that can be used to either play the whole song or play the current pattern only. Depending on which one you assigned to the space key, it will either set (shortcut names starting with "Play Song ...") or unset (shortcut names starting with "Play Pattern ...") the "loop pattern" checkbox. It's always been this way.