Wan's conjecture on limiting Newton polygons

Let $f$ be a non-constant monic polynomial in $Q[x]$. A global permutation polynomial (GPP) over $Q$ is a polynomial $P(x)\inQ[x]$ whose reduction modulo $p$ is a permutation of $k_p$ for infinitely many finite places $p$. A polynomial $g(x)$ is a composition factor of $f$ if $f=g\circ h$ for some polynomial $h$. Wan's conjecture. The polynomial $f$ contains a GPP over $Q$ of degree greater than $1$ as a composition factor if and only if

$$\lim_{\mathfrak p\in\Sigma_{Q}} \operatorname{NP}_{\mathfrak p}(f)$$

does not exist. This conjecture relates arithmetic decompositions of polynomials through global permutation polynomials to the existence of limiting $q$-adic Newton polygons of their exponential-sum $L$-functions; the supplied text gives no evidence resolving it.