Hi, it's a normal feature, I'll explain:

In order to prevent the engaging of the first gear to damage both chain and sprocket, a set of rubber wedges called cush drive, is fitted inside the rear hub, and it is designed purposedly to cushion the impact of a gear being slammed in or a clutch released too briskly. Which is why you see a movement.

The cush drive will wear pretty quick on these bikes and it is generally changed at every service, the 4 sets of rubber only cost about �19 ex vat.

However if you make a search in the forum you will see that people have reinforced worn cush drive by cuttng sections of tyre tubes and filling the gaps to prolong the life of the cush drives.

Here is a page from the official Service Manual.