The glib answer is 'it is complicated'. You have chosen a specific shape with a specific style applied to it. And your document has a theme and a variant applied as well. Then you have local "Format Shape" settings which are also interacting to set the look of the shape.

The simple way to solve the problem is to use a shape which does respond to your shape's format settings. Or change the Theme and Variant on the Design tab and see what happens.

The more involved way is to look at that shape's ShapeSheet and look at the Geometry settings and see why the theme is having an effect. If you can't work out the particular setting that is protecting that corner of the shape, you can load a sample document here so someone can take a look at the variables.

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Andrew Lockton
Chrysalis Design, Melbourne Australia