A little strange to write up a bug hunt that was resolved by the ffi upstream already, and not by the hunt itself. OP didn't fix the bug, though identifying that the upgrade was relevant is of some interest. Writing could have been clearer.
15 or so years ago I had a similar journey - a single python interpreter "impossible" segfault in production that turned out to be a bug in glibc realloc, that had already been fixed in an update, we just didn't figure out to even look for one until we'd narrowed it down that far. (We were shipping custom Debian installs on DVD, a fair number of our customer installs weren't internet accessible so casual upgrades were both impossible and unwanted, but it was also a process mistake on my part to not notice the existence of the upgrade sooner.)
Never wrote it up externally because it was already solved and "Debian updates to existing releases are so rare that you really want to pay attention to all of them" (1) was already obvious (2) was only relevant to a really small set of people (3) this somewhat tortured example wasn't going to reach that small set anyway. (Made a reasonable interview story, though.)
