Unfortunately autocorrect on iPhones at least has been garbage recently. It used to fix words as they were typed, and underlined in red if it couldn't guess, to completely rewriting more than one word going back and giving no visual indication that it happened. This is especially problematic when texting quickly/casually without perfect grammar and it ends up butchering lots of words.

Oh and it's even worse than that if the keyboard is set to support more than one language, where autocorrect will just do whatever it wants.

In the end I just turned autocorrect off on my phone.

The correction bar is a bit better, but it likes to reorder words and often lags even on my new iPhone 17PM.