I wrote out exactly how it happens in my first post.

I type out the formula first, then, with my cursor at the end of the
formula, I add parentheses from the right side of the formula to the left
side. This means I add closing parentheses before I add opening parentheses.
As soon as I cursor backwards past the closing paren, Excel triggers its
"invalid formula" function.

This is how I've entered formulas in excel since windows 3.1, and it's how I
think and see formulas. No previous version of excel has kicked up a fuss
about it, only now they're trying to get "smarter", and they're completely
bolloxing my productivity.