I'm not sure why this is marked as resolved.
The attribute that is rendered by SP Page Builder is wrong.
SP Page Builder renders the below:
<video class="section-bg-video" autoplay="" muted="" playsinline="" looploop="" poster="/forums/poster.jpg"><source src="/forums/video.mp4" type="video/mp4"></video>
The generated markup just needs to be fixed for stored MP4s from "looploop" to just "loop":
<video class="section-bg-video" autoplay="" muted="" playsinline="" loop="" poster="/forums/poster.jpg"><source src="/forums/video720.mp4" type="video/mp4"></video>
It's an obvious bug, which *did* work in older 3.6.x versions of SP Page Builder.