I have a Galaxy S8 Plus. I have an S6 too. I moved from the S6 to the S8+ and immediately noticed that my saved words that I migrated over using the account feature weren't working correctly. I tried everything and nothing worked. But as time has gone on, the predictions and random words generated are getting more bizarre and seemingly random. I've uninstalled, reinstalled, but still nothing.
There is another annoyance and that is the random resizing of the keyboard. I have it set to the largest and again, seeming for no reason, it becomes the second smallest size (I think) and will not change back. It does, allow me to make it smaller. The first time this happened, I had auto screen rotate turned off and set to portrait and I was able to get the correct size keyboard back by enabling auto rotate. This time, no amount of toggling the auto rotate seems to help. My keyboard is too small for my fingers and it is causing many typiing errors, but that is made infuriatingly worse by the fact that the predictions for the spelling errors are seemingly random words that have no bearing at all.
FInal gripe is not being able to turn off the pink wiggly line thing when typing. That is very annoying.
Things are broken and have been for a while now, please don't make me go back to the Samsung keyboard! Thanks (y)