This is the sort of thing you can't help but run into when looking up ffmpeg incantations over the course of building a solution.
Just look how easily you ran into it when you weren't even looking for it.
For such a complex tool, the internet (like StackOverflow answers) ends up becoming a recipe book of practical solutions that would be hard to derive even if you sat down with the documentation and read it front to back. Nothing wrong with embracing that.
For folks who might not realize you can fix this post-encode, you can do this: