Having keystroke issues with macOS Sierra on two different Macs
A friend of mine, who works at an Apple Premium Reseller, told me that:


It's a known problem that has to do with MacOS Sierra;
Results in all different sorts of weird behaviour until reboot;
Only happens on factory installed Macs (running MacOS Sierra);
Is not harmful for your computer (hardware); and,
Advised to wait for a software update from Apple.