You can use Mediainfo (opensource) to check the container and codec of the video that is causing such an issue and compare with the similar one that works.
Thank you for noting that program. It is very useful.
It looks like it is likely due to it being h265 and my using a Firefox-based browser: https://bugzilla.mozilla.org/show_bug.cgi?id=1332136
"Mozilla currently has no plans to support H.265. Our focus will be on AV1." "We will not support h265 video while its patent encumbered."
I had re-encoded it to h264 as I selected my standard conversion profile and did not notice that it was originally h265. That is why the one I uploaded played fine in-browser and locally for me on Windows and why yours played fine locally for me but not in-browser.
Apparently I've somehow managed to have never encountered an h265 video online while using this browser.
my guess is it's the website linked in this post that does this.
(post is archived)