Weird issue while printing an angled side

Hello,

My printjobs have a weird deviation when printing an angled side using supportmaterial.
These items where printed vertically, and as you see the bottom angled side don’t look as it should be. I have this problem for several items already. But always in the same situation: angled side supported. When I have a horizontal side supported its as it should be.
Anyone any idea what this could be?