This seems to be the situation for all UK consignments. I contacted Post.NL about it and this is what they had to say:
“Hi Macario, your parcel is a so called gateway package. Your parcel passes through the Netherlands. However it will not enter our system nor any of our sorting centers. Therefor I do not have any information about your parcel. It is best to contact the sender about the issue. The sender can contact the post organisation your parcel has originally been sent with. Kind regards, M”
I would advise checking with Royal Mail tracking as it is likely to be the 1st place to track it after this.