Denominator conjecture for modular polynomials in the b'-invariants

Let $b'_i(\Omega):=\theta_i(\Omega/2)/\theta_0(\Omega/2)$, and let $L'_p\in\mathbb{Q}[b'_1,b'_2,b'_3]$ be a polynomial describing the locus of principally polarized abelian surfaces modulo $\Gamma(2,4)$ that are $(p,p)$-isogenous to a principally polarized abelian surface $\Omega$ which is isogenous to a product of two elliptic curves by the $(2,2)$-isogeny $\Omega\to\Omega/2$ and satisfies $\theta_0(\Omega/2)=0$.

Denominator conjecture. The polynomial $L'_p$ is the denominator of every coefficient of the three modular polynomials in the functions $b'_1,b'_2,b'_3$.

The preceding proposition shows that the denominators are divisible by a polynomial describing this locus, and computations for $p=3,5,7$ support the assertion that the same polynomial $L'_p$ is the denominator for all coefficients. The source does not state a resolution status.