How to reproduce:
1. Open scene SampleScene from users attached project "playheadJumpTest20182.zip"
2. Select TimelineHaver GameObject in the Hierarchy and make sure that Timeline window is open
3. In Timeline window move the playhead to frame 50 (for example) and zoom in until you can see the individual frames between frame 50 and 100 (roughly).
4. Hit the timeline window's Play button and inspect how the playhead jumps to frame 70, and begins playing from there.

Expected result: Timeline playhead starts playing from its position when Timeline Play mode is enabled
Actual results: Timeline playhead jumps ahead and starts playing when Timeline Play mode is enabled

Not reproducible with: 2017.1.3p3, 2017.2.2p4, 2017.4.2f2, 2018.2.0a3
Reproducible with: 2018.2.0b3

Note: To reproduce this bug you must import WebVR Assets, this was already done in user's attached project