NoWorries

I had this issue and solved it by turning off a setting something like 'recognise the road you're on'. You may find the slip road isn't the problem, rather that it allows access to a motorway that crosses underneath the one you're on. My guess is that the car was using the sat-nav maps and seeing what it thought was a crossroads approaching so braked on the approach. I haven't got access to my car now but I think I found it in the settings menu for the ACC rather than the sat-nav but I could be mistaken - it wouldn't be the first time 🙄