When a light block is placed above a plant that would otherwise grow, it does not. For instance, when placing a light on top of bamboo sprouts, it does not grow. I haven't checked other plants, but this should effect cacti, sugarcane, vines, glowberry vines, weeping vines, twisting vines, and all saplings, and pumpkin / melon sprouts. This could be fixed by simply putting light blocks under the same category as air, cave air, and void air, or implementing a new datapack block tag.