I just chatted with Samsung support in my country and asked them why my TV updated its firmware overnight despite "auto updates" being toggled off in settings. Here's the chat (translated into English):
"Samsung: Updates may still happen, but will be to fix problems. So if there is a general problem or problem with apps it will update anyway. There is a button where you can turn this off, but as I said, it will update anyway.
Me: Well, then this option is very misleading. The only way to prevent this will then be to take the TV off the net?
Samsung: Yes, if that's what you want. But again, necessary updates will be installed. Not those who come as standard. So it doesn't happen always. It's only when a problem with software is detected by our developers with the current version. If there are no problems, it will not update. You may not notice because you don't use the function that has an error. But again, it will update all models anyway.
Me: What do you mean by "not those who come as standard"? What is a standard update, that one may choose not to install?
Samsung: Standard updates are a later version that are not bug fixes. Those who give you support for new apps and things like that. It has nothing to do with errors, at least."