Text mode does not respect \ensuremath{\mathrm{....}}

[josephwright]

Seen in for example

\documentclass{article}
\usepackage{siunitx}%[=v2]
\sisetup{output-complex-root = \mathrm{i}, mode = text}
\begin{document}
\complexnum{1+2i}
\end{document}

(from an email)

[josephwright]

Not quite right!