I’m with you so far as “next” should always the next occurrence of the day, and maybe in some places it does. But practically it doesn’t work. In every place I’ve lived it works like this: “this week” isn’t a set Monday – Sunday like you suggest, but a rolling seven days. Its Monday as I write this, “this Wednesday” is two days from now, while “next Wednesday” is the following. Same for this vs next weekend. If it’s Friday, “this Monday” is three days away. Rolling seven days.

“This” cannot be used for the day of week you are currently on, nor can it be used for previous days.