Basically what Apple is asking users affected by this bug to do is to reset their keyboard’s dictionary. This can be done by going to your Settings > General > Reset > Reset Keyboard Dictionary. After that you have to reboot your device and it should be back to normal, at least that’s what some are claiming.
The fix seems to be a mixed bag where some are claiming that they have had success with it, while some claim that this does nothing for them, so your mileage may vary, but hopefully you fall into the former camp. We can only assume that a proper fix will arrive in a future update, but when exactly remains to be seen.
It is rather disappointing that iOS 11 seems to be riddled with bugs that appear to be rather simple, and ones that Apple should have caught well early on into development, but either way for those afflicted, we guess you’ll have to wait for the proper fix.