thats normal, 4matic is a front wheel drive with rear that give traction if you lose traction in the front. + front engine so more weight on the front.I just went through this.
my tires have 6mm of tread. When the front was ~2mm, the back was 5mm. That surprised me.
I’m gonna try not rotating and jusr burn 2 sets of fronts per set of back.
4matic can accept easely different wear from front to rear as its not a permanent 4 wheels drive like subaru for exemple.
if you drive hard or go on race track the wear of your tyres will be more balance I think, but always more front than rear.