I mentioned in an earlier post that my "grandteacher", Y. R. Chao 趙元任, did his dissertation on a problem in logic, even though he is best known for his work as a linguist and composer. (Though I think that his Language and Symbolic Systems, 1968, and his methodology in descriptive and historical linguistics all clearly show his predilections.)
I'm happy to say that Mitch Keller, who runs the Mathematics Genealogy Project, has now agreed that both Chao and his teacher, Henry Sheffer, deserve to be listed in the records there, even though their degrees were from departments of Philosophy. Chao's entry is here and Sheffer's can be found from it.
It seems that none of Chao's students has a degree in a strictly mathematical field, though perhaps some will be found in the future.