I have exactly the same issue. When navigating, the GPS switches between my position and a position in the middle of the Atlantic Ocean. It switches between the two positions so often that it becomes unusable. Impossible to use the GPS as it is. At times (always at bad times...), the GPS puts me in the middle of the ocean, and I don't know where to go. So I stop. I tested with the latest version of iOS 18, but also with the various betas, and no improvement.