Car: 9th Gen 1.4 Petrol SE + Navi
Join Date: 13th March 2017
Location: Essex, UK
Thanked 294 Times in 264 Posts
Is it happening in both econ and normal mode?
Econ mode alters some functionality of the cruise control, including being more permissive about slowing (Will allow ~3MPH Below target speed)
May be worth trying in normal mode, and see if that rectifies the issue.
The 9G Cruise control has no connection to the brakes, so any slowing is due to the engine - are you in a cruising gear? Being on a flat, you should only have gentle engine braking in 6th (Well, is the case on 1.4 - I know diesels are usually better for engine braking)
Other than that, I'm not too sure what else it could be.