Hi! It could be a graphical artifact or there is bad geometry. Please press Ctrl+F7. There will be a dialog showing the body quality. Scroll down to see if there is a bad body. Run through Repair Bodies workflow to fix up the badness.
If it is a graphical artifact, make sure graphics driver is updated and all critical Windows updates are installed. After that, if it still happens, it could be a limitation in Inventor that cannot display the perfect section properly. Please feel free to share the example here. Forum experts can help take a look.
Many thanks!
Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer