Subtle retina pinstripe after iOS 18.1.1 update

Since updating to ios 18.1.1 my retina display has developed barely visible uniform stripes across it at regular pixel intervals. They cover the whole screen uniformly and are so subtle I initially thought they were a pinstripe background effect. They are not there right now, but they were visible when the battery was in the red a few minutes ago. They were visible in all apps, particularly on a white background. (It seemed to be software induced rather than the kind of symptoms I would recognise as failing hardware.) Is this a bug, or is someone trying to save power by displaying grey pixels instead of white ones?