Recently I found that embedded videos don’t appear on Twit.tv pages using Safari on my MBA '18 using Mojave. I have the same setup, including ad-blocker Wipr Safari extension from the App Store, on my Mac mini, but they show up there normally. I’ve checked Safari website prefs and cannot see any difference.
Any recommendations for tracking this down?
In the event anyone else has this problem, a restart to Safe Boot corrected this. 
I’ll bet simple restart would have worked. 
Actually, it’s a bit more complicated than that.
Changing “Settings for this Website” to “Never AutoPlay” will stop the video frame from loading. Changing the setting back and restarting Safari will allow the frame to load on the page. And only on my 2018 MacBook Air. Doesn’t seem to do this on two other Macs, one on Mojave and one on Catalina. ^2