It is water between the seals, not in the door cavity.
I really do not think it is acceptable as normal however, and that is based on the volume. I understand water will reach that area, as it does on any car. However, it should not be trapped there in quantity. Opening the door causes a small flood to pour out. Yes, that is a bit of hyperbole, but the amount is not insignificant, and if you could catch it all likely would be equial to at least a couple full measurable cups worth.
Normally the doors should be engineered to allow this water that gets in there to also get out and be released on its own. Not hold it all until the door is open.
For our latest dealer visit, we had a loaner 2017 which did not do this for instance. Nor has any other car I have owned, and it is a long list.
It goes in again tomorrow, to a new dealer, so we will see how it goes. But really, this particular MKC while being an amazing ride and drive, has been a real disappointment in build quality and reliability.