The phone can't correct what you're typing if it doesn't predict what it is. As you type it's predicting the word, hence it will autocorrect it with the closest match to its predictions.
I been an IPhone user for years, and now recently switched to the Galaxy s8 since my intelligence grew but I still remember every detail in the ios since I study phones as a hobby, and I used autocorrect without predictive text. It's annoying seeing the predictive text bar (I meant the bar if you didn't interpret the bar, my apologies if so).
Ah. Ok. The bar settings differ on a per keyboard basis. Depends on the app if can be turned off or not. You may have to download an app that has autocorrect but turns off the prediction bar, having prediction/autocorrect working in the background.