The terse description for PlaybackState.Begin is:
The tween is beginning to play
Based on my observations this is incorrect and does not accurately communicate how tweens behave. This PlaybackState actually has nothing to do with playback. A better description for this PlaybackState would be something like:
The tween has just been created and has not been played yet. Once this state has been exited, it will never be entered again.
Page URL: https://create.roblox.com/docs/reference/engine/enums/PlaybackState