Can you link the video you’re trying to embed? This might be a temporary issue on Wistia’s side… when looking for a video to test I found this link, which doesn’t seems to load a video, just a blurry thumbnail:
The rich text and video element versions use Wistia’s iframe player, which loads an iframe like (Fallback Embed Final Proof 1) … their HTML embed codes load some javascript to load the video element into the page. I think it was their bug, but only in the iframe version.