The entire leaf is gone with only the veins left. Unable to see any insects on stems or flowers. Flowers are not affected.
Something Is Eating Leaves on Knockout Roses
Sounds like a worm of some kind (likely the rose slug) that is feeding upon the roses. Here is a link to an article on them for you: https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-slugs.htm I recommend spraying the rosebushes with an insecticidal soap for quick help and then perhaps give them some Bayer Systemic rose and flower care granules. I do not like using the systemics, however at times a treatment is needed to gain control over a formidable foe.