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Not only there. Automatic word replacement is my absolute least favorite iOS mobile feature. I cannot tell you how many times I painstakingly, painfully thumb typed exactly what I meant just to have it changed out from under me. And I have gone back to see nonsense messages where I know I didn't type what was sent. Very frustrating.
I think this could be corrected if the spell corrector factored in the time taken to write the word out. If the user takes longer than average to write the word they are probably deliberately spelling it out, compared to just mashing the screen to get the words through.
You could even train it on past messages making it more tailored towards particular user.