I've had a couple of reports of the keyboard arrow keys failing to work.
   24th July - worked OK the day before.
   2nd Aug - at 6pm failed. Worked OK an hour earlier.

My web page did not specify the version, like ...api/js?key=AIz...
This fixed the problem: ...api/js?v=3&key=AIz... 

The documentation indicates that by not specifying a specific version I was calling the v=3.exp version.

I have reproduced the problem myself today, 5 Aug 9:50am

 no version (default)  keyboard arrows do not work.
 v=3.exp               keyboard arrows do not work.
 v=3                   keyboard arrows work OK.

I don't know if other people have noticed this or that the people working on the v3.exp version know about it.