Is it common for the armrests to touch the back when folding down?

The Plus models are supplied with 5D armrests, which are able to to move the arm pads forwards/backwards, left/right and also turned in. In certain settings, it is quite normal for the armrests to touch the backrest cushion when they are folded down.

You can check how the armrests are mounted on the seat cushion - if necessary, you can move the armrests a little bit and reattach them to get a little more distance.