That's bizarre. Haven't experienced that myself and I don't know anyone else with that issue on their S6. Try hard reboot and see if that clears the bug.
I have had it randomly happen I have actually started restarting my phone every 2 or 3 days to clear up weird issues. I really should wipe it but I am very happy with the way it is set up so I have not :-(
The only thing I can think it would be was a bug on one of the samsung themes?
I'm using the 'urban' one (like the app icons best) with my own background.
Like i said it only happens when I'm interupted when using the keyboard and the phone locks!!