I would guess it's probe location. You're in the center of the space, and the cooling is coming from the walls. By the time it hits 63 in the center the walls are much colder, so it drops further as the heat evens out in the space.
First, put some stuff in it - an empty space like that makes the compressor work much harder. At the very least put some water jugs in there. The more stuff in the space the more it will even out.
Don't put the probe in water to begin with, that will just make it worse because you're waiting for the freezer to suck the heat out of the walls, which pulls it from the air, which pulls it from the water until the water gets cold enough. That's going to take a long time and the walls will be even colder.
Basically, put stuff in it and leave it alone. Given time it will even out and it will work as expected.