New Bath shower mixer - diverter won't stay up

Replaced out an older bath shower mixer tap with a this new Sonas one that claims it works from .1bar (I have gravity fed going to it).

The old tap was connected with narrow bore flexi's and also had an extra un-needed coupler. I removed the coupler and also used these full-bore flexi's - albeit they have an isolater built in that would reduce bore.

Problem is everything works as expected except the shower diverter won't stay up, I have to hold it constantly up. The old diverter stayed up under this pressure but the flow was pretty poor.

I'm guessing it's either not enough pressure for this new tap despite it meant to work off .1 bar or else possibly a faulty diverter?

Any advice on what it might be appreciated.