I realise this may probably boggle the mind of the modern software developer.
If there is a memory leak, them this is a flaw, that might not matter so much for a specific product, but I can also easily see it being forgotten, if it was maybe mentioned somewhere in the documentation, but maybe not clear enough and deadlines and stress to ship are a thing there as well.
Most of the time QA can tell you exactly how the product works, regardless of what the documentation says. But many of us haven’t seen a QA team in five, ten years.
At some point you have to admit that humans are pretty bad at some things. Keeping documentation up to date and coherent is one of those things, especially in the age of TikTok.
Better to live in the world we have and do the best you can, than to endlessly argue about how things should be but never will become.
Shouldn't grey beards, grizzled by years of practicing rigorous engineering, be passing this knowledge on to the next generation? How did they learn it when just starting out? They weren't born with it. Maybe engineering has actually improved so much that we only need to experience outages this frequently, and such feelings of nostalgia are born from never having to deal with systems having such high degrees of complexity and, realistically, 100% availability expectations on a global scale.
The amount of dedication and meticulous and concentrated work I know from older engineers when I started work and that I remember from my grand fathers is something I very rarely observe these days. Neither in engineering specific fields nor in general.
What works much better is having an intentional review step that you come back to.
Now, there can be tens of thousands of similar considerations to document. And keeping up that documentation with the actual state of the world is a full time job in itself.
You can argue all you want that folks "should" do this or that, but all I've seen in my entire career is that documentation is almost universally: out of date, and not worth relying on because it's actively steering you in the wrong direction. And I actually disagree (as someone with some gray in my beard) with your premise that this is part of "rigorous engineering" as is practiced today. I wish it was, but the reality is you have to read the code, read it again, see what it does on your desk, see what it does in the wild, and still not trust it.
We "should" be nice to each other, I "should" make more money, and it "should" be sunny more often. And we "should" have well written, accurate and reliable docs, but I'm too old to be waiting around for that day to come, especially in the age of zero attention and AI generated shite.