Yup.. it is Ray Epsilon. Changing its value changes the size of the hole (line) BUT.. the inner hole remains with partial shadow when it should be 100% shadow.
And im not certain if the changes to alleviate the line will create problems with the rest of the render.
so its definitely a glitch , unless another function value will remedy the hole.
Now.. i understand that this shouldn't be much of an issue considering it a shadow being covered up by the object. But when compositing if your to manipulate the shadow its possible that the object will
be in a position that the presence of the hole will create issues.