I am trying to find the extremes of this curve in the XY plane… and failing miserably ( The docs say nothing about the requirements of the input curve having three dimensions

The curve has no Z value. its a 2D curve that sits ON THE XY plane, so I expect the XY plane to be the correct choice

I don’t know, tolerance issue maybe. I opened the file in millimeter template with tolerance set to 0.001 and your gh file returns 4 extremes, as expected.