Sometimes autocorrect will "correct" perfectly valid words if it deems the correction more appropriate. Ironically while I was typing this message, it changed the word "deems" to "seems" repeatedly. I'm not sure what's changed with their algorithm, but this appears to be far more heavy handed than it used to be.
SwiftKey has this really frustrating one where you'll remove the incorrect word and try again, and it reinserts the wrong word plus something additional.
