12 is stable - we released it after extensive testing. We couldn't find any more big issues. After release, we discovered some (broken) clients had trouble with the CalDAV/CardDAV support so we delayed rolling out updates to users on the Stable channel.
There is absolutely no way to ensure something like that NEVER happens, this is software - there will be bugs. If not in our software, then in other people's software, like in this case.
So if you want the latest stable: nextcloud.com/install
The fact that we have not rolled it out to users is simply because we want to avoid this problem for them. We did not consider the problem important enough to remove our 12.0.0 release, though, new users will set it up, MIGHT find out there's this problem - which will be fixed in 12.0.1. Or people update manually and find out - also not a huge deal.
Is this arbitrary? Yeah, it is a bit. Generally we try to push out the .0.1 release within 2 weeks, so it isn't a big deal. We were slower this time due to a rather sudden big influx of customer work so we haven't merged everything we'd like to have in the bugfix release... We'll certainly make sure this won't happen for 13.