Vauxhall Astra astra 2013 sparts tourer auto gearbox jolts once stopped at lights and left in drivee when foot is taken off brake

My 2013 Astra sorts tourer J type Auto Gearbox when left in drive and stopped at lights, when taking foot off the break sometimes the cat jolts forward, as if the break is still in contact. However if i flick into nuetral first there is no Issue.
I have just had a general service yesterday and the brakes are fine.
So now i propose to have a gearbox service, changing gearbox fluid etc. Mechanic mentioned the banding may be slipping.
Any comments anybody please.