Nested tblr breaks vertical alignment

[nihil-admirari]

In the following, both columns should be header-aligned

\documentclass[paper=b5]{scrartcl}
\usepackage{tabularray}

\newsavebox\SomeNumbers
\sbox\SomeNumbers{\begin{tblr}{hlines, vlines}1 & 2 & 3\end{tblr}}

\begin{document}
  \begin{tblr}{colspec={X[l, h]X[l, h]} , hlines, vlines}
    {Fill \\ some \\ boxes} & \begin{tblr}{hlines, vlines}1 & 2 & 3\end{tblr} \\
    {Fill \\ some \\ more \\ boxes} & \usebox\SomeNumbers
  \end{tblr}
\end{document}

but they aren't

Footer-alignment suffers from the same problem. As a workaround, saving nested tblr is a box restores the proper alignment, as the example shows, but nonetheless.