Use correct timing offsets for animations & keyframes in scene strips in the sequencer that are not placed at frame 1. See T49261 and T49658.
Based on the patch from @Alberto Mardegan (mardy) (D2558), I have updated it to apply against current master, and set the option by default when adding a new scene strip so that the correct behaviour is used without the user needing to tick any boxes (but still maintaining compatibility with old files).