Captivate html5 only plays as scorm, not as xAPI - Why?

Hi - I have a captivate project with a video inserted from vimeo, and a learning interaction on top as an overlay. I did a "Publish for Devices," as HTML5 and xAPI. But when I insert this into a lesson in a course in my LMS (LearnDash, using a wordpress plugin called Tin Canny Uploader, and try to then play it, I just see a black box, yet the "Play" indicator is moving, as if the video were playing. Then it stops at the right time and shows the Learning Interaction. I can click on that and it behaves as expected. But then when I click the "Play" button again, the black box returns, and the same behaviour on the play bar, where I can see the play bar progressing as if the video is playing.

However, when I upload an html5 scorm version of the same captivate file to my LMS, the video plays just fine.

Any advice appreciated - this is very puzzling. 

Kelly