I have small yellow spots on my rose tree leaves - smooth and flat, no pustules and the leaf is healthy looking and dark green.
Yellow Spots on My Rose Tree Leaves
This could be caused by little beetles that probe into the leaves with their snout. It could also be the very beginnings of a nutrient take up problem starting. If the tree rose is in a pot, it could be signs of an over watering problem that is about to get worse or a root bound problem that is about to get worse and transplanting to a bigger pot or into the ground is needed. Here is a link to an article about rosebush diseases to keep an eye out for too: https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-bush-diseases.htm
I have seen this happen before when water droplets from some sort of spray or even watering system have set upon the leaves too long, it is the leafs reaction that it did not like whatever the substance was but it was not enough to cause any major problems. Keep an eye on it to see if there are any changes.