One of the functions of the dark night, which comes some time after a clear (or near-clear) and stable awakening to selflessness, seems to be to burn through and out (most of ) the remaining traces of a sense of separate I, and the effects of a sense of separate I on the personality...
The second part of it, burning through the effects of a sense of separate I on the personality, seems crucial here.
For most of us, our personality is formed within a sense of separate I. And although it does reorganize to a certain extent following an awakening, many traces of this sense of a separate I still remains.
And the dark night is one of the ways this is burnt through, allowing the personality (and the individual) to reorganize more fully within a context of realized selflessness. Maturing into it, becoming more seasoned.