I have used maternal incubation with numerous species of python eggs, including ball pythons, and have never seen this occur even in 95% to 100% humidity. However, it will occur with the eggs of any python species if your substrate is saturated and too damp, even in a lower humidity. The main issue here is that it is only one egg. If the substrate were too damp, all or most of these eggs would show a water line on the lower half of the egg surface.

Kelly