libertinus is not compatible with bm

[Firestar-Reimu]

\documentclass[a4paper]{article}

\usepackage{amsmath}
\usepackage[math=+ss08]{libertinus-otf}
\usepackage{bm}

\begin{document}

\[
    \bm{k}
\]

\end{document}

This produces an error:

Extended mathchar used as mathchar (14799960).
<to be read again> 

Does libertinus-math have no Math bold-italic fonts like these?

[hvoss49]

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage[math=+ss08]{libertinus-otf}
\begin{document}

\[
    k\symbfit{k}
\]

\end{document}

[Firestar-Reimu]

So it is because libertinus-otf uses unicode-math inside so cannot use bm?

[hvoss49]

Why should it use bm? All glyphs can be bold by optional setting.