root motion is offset after scrubbing back & forth through a blend in Timeline

repro:
- open attached project
- open OGbug scene and select character
- open and scrub the timeline back & forth
bug: the character's initial position is different (moves forward) after every scrub.
workaround: disable GO optimization on the Animator component... scrub again... root motion plays as expected.

this is a regression.
Not repro in…
Version 2018.3.0a3 (0ac1bca97520)

Repro in…
Version 2018.3.0a4 (35209f6de348)