Conjecture on Cohen–Macaulayness and exceptional parameters for two-part arrangements

Let $s$ be a positive integer and let $b$ be a complex parameter. For the partition

$$\lambda=(b+1,b,1^s),$$

let $Y_\lambda$ denote the associated quotient arrangement. Generic Cohen–Macaulayness conjecture. The variety $Y_\lambda$ is Cohen–Macaulay for generic complex $b$. Its exceptional values are

$$b=0\quad\text{{\rm and}}\quad b=\pm\frac{p}{q},\qquad 1\leq p\leq s+1,\quad 1\leq q\leq 2.$$

These are asserted to be the same exceptional values as for $(b^2,1^{s+1})$. The source presents this as a conjecture; its general status is not resolved there.