Plums have a large brown spot underneath skin

So after 5 or 6 years of no fruit on my Beauty Plum, now have about 80 fruit on my tree. The first 3 plums that looked ripe all had a large brown area under the skin. The outer skin is perfectly shiny and unblemished but you can see how brown the flesh is underneath.

Is this from some kind of bug that got into the fruit when it was small or is it some kind of mold?

Another angle:

Can you cut it open to see what it is like inside?

I did cut it open and it is just brown underneath and soft. Next one I see I will post a picture.