Here’s a list of paint bugs and glitches that I know of in Horizon 4. If you know of others than list them below.
Huayra BC. Cannot be painted with decals on the entire carbon part of the car. This has been an issue since Horizon 3.
Centenario. Cannot be painted full Gloss Carbon or full matte carbon.
Paint bugs. When a car is painted and decals of the same color are placed on the car than the outline of the decals is visible.
Zonda R. Cannot be painted gloss carbon. Both gloss and matte will make the car matte carbon
Koenigsegg One:1 cannot be painted matte carbon. When painted matte carbon only the mirrors will be matte carbon.