Sierra Beta: External keyboard won't type "a" and types an "i" instead of "h" after several seconds of being plugged in
Solution 1:
EDIT: I updated my machine to the latest Beta and I also got this issue. I resolved it by going to:
Preferences -> Keyboard -> Modifier Keys...
And then "Restore Defaults".
This is a known issue in latest Beta.
Solution 2:
As noted by others here, doing a "Restore defaults" on the modifier keys solves the problem (System preferences > Keyboard > Modifier keys...).
I was having the same problem - I restored defaults and the keyboard started working fine :)
Note: Re-configuring your custom modifiers after you restore defaults will not re-introduce the buggy behaviour. i.e., once you've restored defaults, you can go ahead and re-customize your modifiers as they were before.
Happy clacking!