Animation Editor ignores the first key in the timeline if there are no more keys for that particular curve.

e.g. I want a camera to start an animation from a different position to where it is in the scene. So I paste that position into a new animation clip at timeline 0:00 . This moves the camera instantly in the viewport.

If I click the play arrow for timeline playback (not game playback) it all works great until I click it again which the camera will revert to its in scene position rather than the key at frame 0, scrubbing the timeline does not fix this, you have to re-enter the values again at frame 0:00.

To solve this you have to add a secondary key for position even if you don't animate the position it needs a second key in the timeline somewhere else it ignores that first key.