From what I've heard (FWIW), Airbus released a version of the software for one of the flight computers that removed SEU protections (hence grounding affected models until they could be downgraded to the previous version).

There was still hardware redundancy though. Operation of the plane's elevator switched to a secondary computer. Presumably it was also running the same vulnerable software, but they diverted and landed early in part to minimize this risk.

So not just redundancy but layers of redundancy.

Why would you ever expect one bit flip? You have a flip rate and you design your system to tolerate a certain bit flip rate. Assumptions made during requirements establishment were wrong and nature eventually let them know they had negative margin.

Possibility of bit flips from cosmic radiation only really came to fore in 1990s, and some aircraft and parts predate that.

  if (cosmic_ray) {
     do_not_flip_bits()
  } else {
     flip_away()
  }