Help with multiplex config

I have a chain of 3 P8 (40x20 ) type display
When using the multiplex option P8Outdoor1R1G1BMultiplexMapper
it displays beautifully but the order of the panels is incorrect.

sending 1234567890AB
gives output 90AB56781234

the cmd I used
./scrolling-text-example --led-brightness 20 --led-gpio-mapping=adafruit-hat --led-cols 40 --led-rows 20 --led-chain 3 --led-slowdown-gpio=5 --led-multiplexing 16 -f …/fonts/10x20.bdf “1234567890AB” -s 0

without the -S0 switch it scolls across panel 1 , then panel 2 and finally panel 3 ( should start from panel 3)

I have no idea how the multiplex-mapper code works.
So I was after some help either on how to map or even an explanation of the code.