Merged table head cell has wrong width with pandoc 3.1.2

[zwz]

Here is an MWE:

<table><thead><tr><th colspan="2" align="center">long text in a merged table header cell</th></tr></thead><tbody><tr><td align="center">1</td><td align="center">first line</td></tr><tr><td align="center">2</td><td align="center"></td></tr></tbody></table>

the resulting latex output:

\begin{longtable}[]{@{}cc@{}}
\toprule\noalign{}
\multicolumn{2}{@{}>{\centering\arraybackslash}p{(\columnwidth - 2\tabcolsep) * \real{0.0000} + 2\tabcolsep}@{}}{%
long text in a merged table header cell} \\
\midrule\noalign{}
\endhead
\bottomrule\noalign{}
\endlastfoot
1 & first line \\
2 & \\
\end{longtable}

The width (\columnwidth - 2\tabcolsep) * \real{0.0000} + 2\tabcolsep makes the head cell very narrow.

Before updating to pandoc 3.1.2, I have pandoc 2.12. It has no such issue.

[jgm]

@tarleb you may want to have a look at my fix for this.
It seems to do the right thing in various tests, but further testing might reveal something I missed.