That's not a bug ! Only the recents parts (and few exceptions) made with the new design system can be fully modified. So, the older one will have this weird lines due to the former way to make decals

Not much to add. There are limits with the real time algorithm to auto convert old textures to the newer version of the part system. There was a time when the background of decorated pieces could not be changed.

i dont have the color selection panel
😦