  In 2023, Kuznetsov and Speranski introduced infinitary action logic with
multiplexing $!^m\nabla \mathrm{ACT}_\omega$ and proved that the derivability
problem for it lies between the $\omega$ and $\omega^\omega$ levels of the
hyperarithmetical hierarchy. We prove that this problem is
$\Delta^0_{\omega^\omega}$-complete under Turing reductions. Namely, we show
that it is recursively isomorphic to the satisfaction predicate for computable
infinitary formulas of rank less than $\omega^\omega$ in the language of
arithmetic. As a consequence we prove that the closure ordinal for $!^m\nabla
\mathrm{ACT}_\omega$ equals $\omega^\omega$. We also prove that the fragment of
$!^m\nabla \mathrm{ACT}_\omega$ where Kleene star is not allowed to be in the
scope of the subexponential is $\Delta^0_{\omega^\omega}$-complete. Finally, we
present a family of logics, which are fragments of $!^m\nabla
\mathrm{ACT}_\omega$, such that the complexity of the $k$-th logic lies between
$\Delta^0_{\omega^k}$ and $\Delta^0_{\omega^{k+1}}$.
