Indeed. The behavior I've seen consistently is that the first N seconds play, then buffering starts, and it _never recovers from buffering_. And it has been this way for at least a year. Astonishing.
Without fail, my experience is:
Play video. Usually works.
Try to replay video. It stops at 4 seconds.
Refresh browser. Now the video is half the size? But it now plays, though tiny.
I don't know who wrote Reddit's video player, but it's obvious they are a masochist. That they still haven't fixed it speaks volumes of their eng org to an end user like myself.
It's why on the Desktop, at least, I always have a https://redditsave.com/ tab opened. It's just way faster (and less frustrating). There's an FF extension that automates this : https://addons.mozilla.org/en-US/firefox/addon/downloader-4-... : which I haven't tested.
*update, a few minutes later: decided to install extension. Looks useful. Now the testing phase...