Sorry, I wasn't exhaustive in my tests. It turns out this fixed behaviour for PHP 7, but broke it on 5. I'm investigating the difference.

The failed build is due to sort stability having been changed between 5 and 7. I'll send a diff to revert this, since phabricator doesn't support 7 officially. (T9640)

It's possible the tests are incorrect, although it would be good to understand why PHP5 and PHP7 differ.

One possible clue is T8929, which appeared to have some non-obivious behavioral difference in preg_match(). Remarkup uses a lot of regexps -- this is grasping at straws a bit, but is the only thing that springs to mind as spooky behavioral differences we haven't accounted for.

Do you know where the offending sort is?

Oh, $rules = msort($rules, 'getPriority');.

Yeah, let's revert this and I'll do the legwork to fix it properly since this is entirely my fault. We have a proper backward-compatible stable sort (msortv()) now but rules shouldn't have ambiguous priorities anyway.