OK, my dopey fault - you did mention the driver board, and i didn't notice...

The switch inputs, IC 15 and 16 are all as you suggested - with high's around 4.86v and lows of 0.06v

The outputs however IC 17 and 18 are the complete opposite of your recommendations ie:

Measured

1L   2H
5L   6H
12H 13L
8H   9L

Thanks