Well, that light, and the two white switches are two different things.
There are two functions known as A/T in the 787.
One is the standard control for the autothrottles, which are the two white switches.
There is no light which comes on when you turn them on.
You know if they are on or off by the switch position.
The A/T light/button is automatic throttle for speed control, which is actually “airspeed hold”.
This is activated by pressing that light/button, which is why it doesn’t come on when you activate the two upper switches.
I really can’t explain why it sometimes does and doesn’t work.