Originally submitted by @sugoisugoma

When you open level in level editor, time cursor is positioned slightly before the start. If you play preview from this position, the level will start from the final video, or will just throw the player out of the level if there is no video. Also playing the other levels from level selection will cause them to behave the same way.

Scrolling will fix the cursor though and it won't fall behind the starting 0th beat ever again.