Cossey's conjecture on lifts of irreducible Brauer characters

Let $p$ be a prime, let $G$ be a $p$-solvable group, and let $\varphi\in\operatorname{IBr}(G)$ be an irreducible Brauer character. A lift of $\varphi$ is a character $\chi\in\operatorname{Irr}(G)$ whose restriction $\chi^0$ to the $p$-regular elements of $G$ equals $\varphi$, and let $L_\varphi$ be the set of all lifts. If $Q$ is a vertex for $\varphi$ in the sense of Green, Cossey's conjecture.

$$|L_\varphi|\leq |Q:Q'|.$$

This conjecture generalizes the known bound in the normal-vertex case and is established when $Q$ is abelian, but its general status is not specified in the source.