I think you‘re giving big meaning of a very regular word: mission.
It comes from Latin and transfered to English through French. James Bond has missions but your kids too.
Your kids often have the mission to tidy their room. It’s ok if it’s not very important.
Having a mission doesn’t mean having a life criticial mission.
So having a mission critical bug doesn’t mean people will die. It’s just mean that this bug prevent your software to do its main job. It’s no big words… It’s the actual definition.
And it’s not you are just an app to order beer delivered at your place that is has to be crappy and not even working.