I have written a new version of `nicematrix` (v. 6.0 of 2021-08-10) which solves that problem (by using the technique used by touhami in its first code. code). It's now possible to use `\multicolumn` in a `{tabular}` nested in a `{NiceTabular}`.

I have written a new version of `nicematrix` (v. 6.0 of 2021-08-10) which solves that problem (by using the technique used by touhami in its first code. It's now possible to use `\multicolumn` in a `{tabular}` nested in a `{NiceTabular}`.