Hey @etah,
Apologies if I missed the broken video part and thanks for testing other image aspect ratios. My recommendation was based on my discovery in my site and maybe you can use an image background to avoid the doubling of the image effect.
Back to the issue, I loaded up your image in my test site and got the same result.
That is because your image is lower in resolution that my monitor. Your image is 1023px X (width or horizontal) Resolution while the monitor I’m currently on is 1366px.

I loaded up a 1400px X Resolution 1:1 image and there’s no double image issue.

I’ve mentioned previously that you need to use a high resolution image.
Kindly try a high resolution image.
Regarding your question:
No. I’ve previously said that the doubling of the image could be a bug BUT, when you showed me that it still doubles up when using 1:1, I realized now that the video poster is trying to compensate for mismatch in aspect ratio and also if the image is low resolution just like the effect in video sites. This is not a bug. You just have to use the best poster image.
With that said, we will not support any custom coding that will be applied to the video background poster nor any custom coding in general because that will be tweaking how a feature of Pro is designed.
Hope that helps and thank you for understanding.