Safari "Failed to load resource: Plug-in handled load" mp4 in lightbox

i added a video from the ALB into a container. added an MP4 that i had uploaded to wordpress. added a preview image, and set it to open the video in a lightbox. when i go to the page and click on the preview image, the lightbox opens and i see a tiny tiny image of the first frame of the video, and i see the playback controls all mashed together in the middle. “sometimes” if i then click the play button, it’ll load up & play (not always).

when looking in the browser console, it says “Failed to load resource: Plug-in handled load” for that mp4 file.
it also says that “sometimes” for the favicon, but also, not always. but i’m more concerned about this video issue.

i compressed the original video in the Handbrake app and made sure to check the “web optimized” option.
and when i just have the video embedded with the same ALB but NOT to open in a lightbox, it works great! i may need to revert to this method if i can’t figure this lightbox problem out.

this only seems to happen in both Safari on macOS and Safari on iOS. no troubles in Firefox or Chrome.

the site isn’t live yet, so i’ll put the access info in the private section in case you need to see it for yourself.
thanks!