I brought this up with Apple Support about two years ago, just before turning off autocorrect permanently. Despite protestations to the contrary by Apple, who insisted that the dictionary is professionally curated, this is clearly no longer the case. The dictionary we all share today is maintained by the entire user base. This means that as more illiterate iPhone users change its to it's and their to there, and iPhones to iPhone's, Apple's Bayesian dictionary allows those errors to percolate to the top, displacing the spelling and grammar rules instituted by the professionals way back when Steve Jobs introduced the first iPhone to the world.
The Bayesian process of maintaining the dictionary also causes us to contribute to the problem. Every time we accept an erroneous word before noticing the error and going back to correct it, we register one vote for the incorrect entry and another one for the correct one. As George Carlin put it, "Artificial intelligence is no match for natural stupidity."
It is saddening to see that this problem has not been addressed yet.