The problem with Mastodon is that some people still don't know how to spell it properly. It's not rocket science.
Mastodon? It's a dictionary word, so it as that going for it—and yet, people see it a hundred times and still often misspell it. The pronunciation's just a bit too far from the spelling, at least in American English. Most folks say it as if it were spelt "mastadon", especially if they're not trying to enunciate carefully & precisely and (perhaps) don't know the correct spelling.
(I'm not weighing in on whether that's some kind of serious problem for Mastodon, but I do think it's worse than all your other examples, as far as likelihood of misspelling even after significant exposure—then again, web search smooths over most of that, for anything sufficiently popular)
You see the same thing with other words that have "o"s that function similarly. You'll hear Americans turning "o" into "uh" in Tyrann-o-saurus, p-o-tato, etc.
Especially if we're talking about an Internet thing: you've seen it in written form more often than you have heard it enunciated out loud. And seems like the iOS text prediction is also able to spell it the right way.
I don't care honestly, but as a non native English speaker I've never understood why natives make an enormous amount of banal spelling errors.