Occasionally I'll find a simple word that's missing from my Samsung Galaxy S8's default keyboard. The most recent example was the word hops. It kept changing it to hopes.
Have I done something silly to make it forget these words in the past?
Most likely situation is that the developers of the keyboard somehow missed that word. You can only add words to the Autocorrect database, not remove words. Therefore, it's not your fault.
It also depends on what words you use more often. So, sometimes the keyboard will still apply its brain and guess that you intended to write hopes even when you have written hops.
You just have to undo that change and it should stop repeating the same mistake again. Also, check your keyboard's prediction settings and the prediction language.