VSTO C#: Weird collapsing problem with shapes

2020-02-15 c# ms-word vsto watermark

I'm adding a watermark in a Word document. I'm calling Shapes.AddTextEffect() in the headers of each section. At first glance it seems to work just fine. But actually a weird thing happends when testing: if I draw a shape by hand and then I copy/cut-paste it, the watermark collapses! Any idea why? Do I have to add a canvas and then add the shape inside?