I don’t know, some autocorrect just suggest the most used word for that set of screen taps. When you tell the autocorrect to remember a word by selecting it, it then just becomes another word to the autocorrect, and the more you select that spelling, the more it’s suggested.
So what I’m saying is that OP has trained their, sorry thier, autocorrect wrong.
Just for that silly sentence above I’ve just had to delete the misspelling and the capitalised version of it from my own dictionary, and had to skip over 3 American misspellings just to write this sentence because I’m British.