Inventor DWGs are not native to AutoCAD, and therefore cannot be simply modified as usual there. The interoperability is great for making titleblocks in AutoCAD and using them in Inventor, but I would suggest you to stay far away from editing Inventor DWGs in AutoCAD as if you could go back and forth. Usually such workflows are destructive to the drawing and only used for deliverables (after release) when you need to submit some changes to specific customers, and the modified drawing in AutoCAD will never see the light of Inventor again.
A little more on Inv. DWG editing in AutoCAD: https://www.cadlinecommunity.co.uk/hc/en-us/articles/201768852-Inventor-Drawings-DWG-v-IDW-AutoCAD
I would guess that your shaded views may have texture from certain Bitmaps attached... try maybe changing this setting from Always to Offline only?
