Reproduction steps:
1. Open the attached project "PlayScheduledBug.zip";
2. Open the "Test" scene;
3. Build for WebGL (File -> Build Settings);
4. Open the produced "index.html" in the directory which you have chosen;
5. Wait for the audio clip to begin playing;
6. While the 6 second clip is playing, click a different window to pause the game (and a clip as well);
7. Wait for a few seconds;
8. Click on the browser to resume the game.

Expected results: When the window regains focus, the game resumes and the clip resumes from the point where it was paused.
Actual results: If the window loses focus for more than a few seconds when it regains focus the clip will restart from the beginning.

Reproduced on: 5.5.3p3, 5.6.3p2, 2017.1.1f1, 2017.2.0b10, 2017.3.0a7.

Note: This does not happen if using AudioSource.Play.