Have you cleared the cache for the keyboard under the keyboard gear reset to default settings, turn off prediction text, under swipe, touch and feedback make sure the character preview is off. You will have to experiment maybe. Do you notice this occurring in all apps? Maybe it is your theme keyboard if you use a theme? Assuming you are using Samsung keyboard. Do you have a floating keyboard maybe choosing default minimal for everything. Make sure you have updated everything in your phone also. That's what I would do.
