Demo switch toggles but modbus slave output does not

Hi All.
I have now got an arduino modbus sensor slave connected to my RasPi running OpenHab with modbus binding.
I can toggle the outputs on the slave, using the demo site - I bound the bath_ceiling light switch to one of the output coils.
I can also see the data going to the slave via modbus.

The problem I have is that if I click on the light switch quickly, it toggles on the screen, but the output signal being sent to the slave doesn’t always match the output shown on the web page.

I’m only using the sitemap and items files from the demo, and at this stage only wanting to see a reliable connection between the light switch on the openhab page and the actual modbus coil.

Can anyone explain what’s going on here ?