Hi, I still use the T9 or 3×4 setting on samsung keyboard, my thumbs are too big for Querty keyboard.
But about 2 months ago after some update (not the one we just had, to version UI 5.1) b4, around July or a bit earlier. My 3×4 has stopped predicting texts as i start to type; in Samsung Internet, Google search bar, lots of search bars on websites! It still works in WhatsApp, messages and a couple of other apps!
This is very frustrating as I've had to revert to using a Qwerty keyboard which I find very difficult. This happened b4 a couple of years ago then after a few weeks it started working again.
I've tried many things on my phone to get this working again but still not.