Multitrack and Waveform

Essentially, yes. Multitrack is only a posh file player (that's how non-linear editing works), and one of the things it can do is stretch files during playback - but it doesn't alter them in the slightest. So when you double-click on the file to open it in Waveform, then it plays normally - because nothing has inherently happened to it. So when you've got something sounding the way you want in Multitrack, you either have to mix it down or export it (essentially the same thing) in order to get a permanent copy of what you set it up to do.