A little known snafu with circular viewports is that it is hard to access their properties.
For instance you might want it to plot with conceptual shading.
It can be done as follows:
Right click on the viewport and choose properties. You will see 2 items (one is the circle) are selected because the little drop down at the top says "All(2)" , next, click the little triangle to show the drop down. Pick viewport.
Now you can see the properties, so choose conceptual for the plotting of the viewport.