When using contour3() to create a 3-D contour plot, a regular 2-D plot is being created instead.

[X,Y] = meshgrid(-5:0.25:5);
Z = X.^2 + Y.^2;
contour3(Z)

fig2plotly(gcf, 'offline', false);

Expected and generated (https://chart-studio.plotly.com/~xarico10/741/#/) plots:

Examples on using contour3() can be found on:
https://www.mathworks.com/help/matlab/ref/contour3.html#responsive_offcanvas