Oort's direct-sum conjecture for Newton polygons

Let $p$ be a prime, and let $\xi_1$ and $\xi_2$ be Newton polygons that are realized in characteristic $p$. Oort's direct-sum conjecture. Then $\xi_1 \oplus \xi_2$ is realized in characteristic $p$. This conjecture concerns whether realizability of Newton polygons in a fixed characteristic is preserved under direct sum. The supplied context says it is an expectation of Oort and that the paper proves several results about it, but does not state that the conjecture itself is resolved.