Agnostic mystic I can believe. Atheist mystic seems unlikely. That's just my opinion.
I've said several times in this thread that terms like atheism, theism, and now agnosticism are irrelevant at the point of mystical awareness. There is no doubt whatsoever that what is experienced is real. What is in doubt is how to talk about this.
Terms like theism and atheism basically apply to questions about
ideas about God. When you enter into the mystical states, there are no ideas going on. It's is not an "idea" experience. Ideas of God are pointers to the Absolute, to the "big question", the "ultimate concern" as Tillich called it. So atheism is an idea of what this is, which says "no deities". Fine. Theism is an idea of the Ultimate that say 'a deity exists'. No problem. But once one is immersed in the mystical state, all ideas are superseded. You are no longer operating in the idea domain. You now know. And God is no more. You don't need an idea of God anymore, be that theistic or atheistic, be that Atman and Anatman.
Both theism and atheism are right. Both are pointing to the Absolute. And once you are immersed in the Absolute, both become non-questions. Both become moot. Here's a fantastic quote from the 14th Christian mystic Meister Eckhart that captures this in the paradoxical prayer he makes,
"I pray God make me free of God so that I may know God in [his] unconditional being".
What this is saying is make me free from my ideas of the Absolute, be that theistic or atheistic. Both theism and atheism are the same thing, not opposites. They are both relative views of the Absolute which is beyond them both. Both are conceptual. And concepts are not Reality. Both views are substitutes, models of the Absolute, and both when held tightly as the truth, prevent one from seeing and knowing the Absolute. So in a sense, yes, there are no atheist mystics. There likewise aren't theistic ones either! But once the mystical is Realized, both descriptive views are accurate, to muddy the waters further.
P.S. I realize there is a lot of clarification that will need to be explored in this, and hopefully someone will wish to hear it. I'll just add here for point of clarification, that theism and atheism are both paths towards knowledge of the Absolute. So a mystic can be one on the path of ascension, and that path can be theistic or atheistic. But at the point of Realization, these ideas are vaporized. Then after the fact, they become complementary ways to speak about "That", or "God" or whatever term you prefer. The mystical path is a path of ascension and descension. From the relative to the Absolute, to the relative, from the many to the One to the many. I really hope someone wants to explore this perspective on this further. I could fill several pages on it.