I’ve had this problem and found a workaround:
For some reason, while the letter keys intermittently dont work in a text field, the COMMAND and TAB keys work.... and for some unknown reason, if I COMMAND+TAB out of the app I was trying to input type, then COMMAND+TAB back in... It works. And then it works flawlessly until the next time it wakes up and suffers again.
Absolutely no idea why this works, but I didnt see this solution posted anywhere else, so I’m sharing it now.
So yea. CMD+TAB out of your app then back in and that works for me every single time.