Biliotti's conjecture on polar actions on higher-rank compact symmetric spaces

Let $M$ be an irreducible compact Riemannian symmetric space of rank greater than one, and let a connected Lie group act nontrivially and polarly on $M$. A polar action is one admitting a section meeting every orbit orthogonally, while a hyperpolar action has a flat section. Biliotti's conjecture. Every such polar action is hyperpolar. This is still an open problem for compact Lie groups of classical type.