Isaacs's positivity conjecture for Lehrer's character-counting polynomials

Let $l_{n,c}(t)\in\mathbb Z[t]$ be the polynomial from Lehrer's conjecture, counting irreducible complex characters of $U_n(q)$ of degree $q^c$. Isaacs's positivity conjecture. The polynomial $l_{n,c}(t)$ is a polynomial in $t-1$ with non-negative integer coefficients. The supplied text presents this as a stronger conjecture than Lehrer's and does not state that it has been resolved.