To reproduce this bug, navigate to a video URL. When the video starts playing, click the pause button in the controls. Once it changes into a play button, click the play button. Playback will not resume.
This sounds like https://bugs.webkit.org/show_bug.cgi?id=167552. I'm not sure this has made it into STP yet. Can you reproduce with a ToT build?
*** This bug has been marked as a duplicate of bug 167552 ***