White primed door, MDF face cracking off timber core

I purchased a few doors recently, White primed with timber core and MDF face, and have noticed that a 1mm-wide crack has formed down the side of one of them where the MDF meets the timber core (see pic here).

Here's a planed-off cross-section of what the door looks like under the paint layer

Now I could be imagining things but I'm pretty sure that the crack is getting longer each day.

I've called the door shop and they are going to send out someone to look at it, but am I right in assuming this can't be repaired and that I should ask for a replacement?

Thanks