@packetpocket,
Thanks for reaching out to us with this.
This isn’t the fist time I’ve seen this behavior we’re currently trying to figure out what the issue is.
It seems that the setting will reset at some point no matter what you try to do. To confirm, if you click the lock icon, then go to Site settings --> Autoplay and set the setting to Block here, it seems that it does, as you describe, revert back to Allow but even so, returning to YT and refreshing the page – autoplay is either blocked or, if still allowed, clicking lock icon, then setting autoplay to Block again in the dropdown will block autoplay.
If you could/wouldn’t mind taking a short screen recording of the behavior, it would help us document the issue and build a case against it so we can get the error resolved as soon as possible.