This is exactly what happened in this case. After finding 2 different superblocks, dmraid decided that instead of actually asking the user for what to do, it should prefer one over another (in this case ddf1 over isw).
Furthermore, it again decided to automatically rebuild the array and fix the seemingly inconsistency between array members without consulting the user first.
All it took for my RAID set to become invalid and me losing my data, was booting up Ubuntu Desktop 18.04 ISO image.
The author's self-restraint is impressive. If this happened to me, I would (a) lay the blame squarely on Ubuntu, (b) never use Ubuntu again, (c) point to this every time anyone recommends using Ubuntu for serious work.
Admittedly it's more of an emotional reaction than a logical one. Linux is traditionally for forensics (at least compared to Windows and macOS) so it's unfortunate to hear that a distro wiped someone's data simply by booting into it.