How do you hide corner mitres in a LayOut Viewport?
In the Default Tray "Layers" section, un-tick the layer "Hidden on Plan". This will remove the corner mitre.
Next, update the scene selected by Right-clicking the scene's tab at the top of the window and choose "Update" and then save the model.
Finally, Update the model reference in LayOut. This should update the viewports automatically, otherwise you will need to update each render viewport accordingly.