Let $K$ be a finite field of characteristic $p$, $G/K$ be a connected, reductive, split algebraic group. Fix some maximal split torus $T$ and a system of positive roots $\Phi^+ \subset \Phi (G,T)$.
Consider a $p$-restricted weight $\lambda \in X_*(T)$: this is a (dominant) weight such that $0 \le \langle \lambda, \alpha \rangle < p$ for all simple roots $\alpha$.
Just like for any other dominant weight $\lambda$, we have an associated highest weight module $L(\lambda)$: this is a rational, finite-dimensional $K$-representation of $G$ characterized by having highest weight $\lambda$.
Question: if $\lambda$ is $p$-restricted, is it true that $L(\lambda)$ is absolutely irreducible? If so, I'd be grateful to anyone who provides a reference.