I take it all of the 'more options' are left blank?
Can other smart apps turn the light off?
I have a similar issue with a Fibaro Dual Relay, but with CoRE (and rule machine)
CoRE / RM can turn the 2nd switch on but not off... I think it must be something to do with the way the DH is reporting it's state to the cloud. By defult, CoRE and most other smartapps won't bother sending an off command if they think the light is already off.
What's weird in my case is that the 'on' state shows correctly in the main ST app but CoRE / RM thinks it's still off!
I never truly got to the bottom of it but I did find a workaround.
Using CoRE I disabled 'command optimisations' to force it to send the 'off' command regardless of the perceived state... This solved the problem but I certainly won't be buying any more dual relays (I'll stick with the single ones going forward!)
I cannot test for the same problem in Smart Lighting as it cannot see the 2nd switch on the dual relay.