Other artists depict the Mandylion. It is usually shown as a cloth, with the disembodied face of Jesus on it. In this, they are not making something up, they are depicting an actual object, which was still extant and occasionally displayed. As such, their depiction of the face could not be drawn from imagination, or loosely derived, it had to be what the face actually looked like. If they painted a full colour face, everyone who knew the real thing would know that they had got it wrong. If they omitted the bloodstains, everyone would know that they were being dishonest in their portrayal of the Mandylion.