I checked the logs (more than a month later.. sigh -- sorry!), and there's no record of scheduling an archive prior to the April 26th -- when you first wrote this post. I can also reproduce the expected behavior -- in other words that a goal that's scheduled to archive freezes up if it derails without automatically recommitting. So I'm guessing that in this case it was in fact user error.
This is definitely the kind of bug that's really useful to report, because there's lots of room for confusion (and us just totally messing things up), since we're enforcing a delay between when you initiate the action and when you get the result you want, and you're not the first user to get surprised like this, which does make me wonder if there's anything else going on with these kinds of cases! (Or anything else we could do to be more clear about the goal state and somehow make it more patently obvious if a goal is or is not going to be archived (and when! -- I think that the current state of things where there's no indication unless you're on the goal page with the "Stop/Pause" tab exposed that the goal's going to be archived is really problematic, but I haven't come up with a solution yet!)