mw1261-mw1265 are running HHVM 3.18.2 now. They're mostly working fine, but mw1265 segfaulted with what looks like a race condition in memory cleanup. I've reported this upstream at https://github.com/facebook/hhvm/issues/7779
Not sure if it's new in 3.18, we might just as well have seen it with 3.12 already, but never really diagnosed it.