qgis - Voronoi / Thiessen polygons doesn't create a part for 1 of 4 input points

I am using QGIS to create Voronoi / Thiessen polygons for 4 rain gauges.

I am having issues with both the QGIS 'Voronoi polygons' tool and the Grass equivalent 'v.voronoi' where one of my 4 points is skipped and only 3 polygons are generated in the output.

As a test I have added an additional point and have found that both tools output a polygon with the correct number of parts (5). Is there a reason it will not work correctly on 4 or less points?

Edit - image showing the configuration of points and the output Voronoi polygons skipping the central point