Although not apparent in the photo, since the subject is dark, there is a lot of color variation in the dog's hair.
There may be a lot of color variation but the transition form one color to the next is very gradual. If it locked up on the dogs forhead, it is because of the white spot.
Think of two pieces of paper, one black, one white, set the beside each other and there is high contrast.
Now imagine one piece of paper as wide as those two pieces of paper where the color goes from white to black gradually.
You AF will lock up easily on the Black/White paper, but not on the White to Black faded transition.
Some camera AF systems have problems with orientation. I recall that some cameras would 'hunt' on a test target with horizontal black stripes on white paper, turn the test target 90 degrees so that the lines are vertical and it locks up fast. AF systems have evolved, but are not perfect.
If I were shooting your dog, I would have focused manually using an aperture to give me the most depth of field and the shutter speed appropriate.
If I was to use AF, I would set the camera to use one focus point and place that point over the dogs eye, or some other area with higher contrast.