Diagnosing leaf holes can be challenging, but inspecting the plant after dark might reveal the culprit. Beetles, earwigs, caterpillars, grasshoppers, and slugs/snails are common offenders. The damage isn't serious with regards to plant health, though, so fortunately can be ignored. The blemish is probably a leaf spot infection, though similarly not one that requires intervention at this point. If you want, you can clip off that lone leaf if it's the only location of infection, though it's not necessary.

Miri

Ok, thanks.  By the way— I was wondering if leaves with holes from insects are better off left on the plant or does a damaged leaf struggle to photosynthesize? If it’s still feeding the plant, I should probably just leave damaged leaves on the plant…?

Leaves damaged from mechanical means (insect or slug chewing, minor wind or hail shredding, etc.) do tend to still function well enough to photosynthesize and continue feeding the plant, though there is always a risk (albeit small) for pathogens like bacteria to enter those physical wounds and cause infections. (Such infections would create obvious symptoms, so if the leaf looks fine, it's probably in good health.) Sometimes minor leaf spot infections can have the same result -- not damage enough leaf tissue to justify removing the leaf. More advanced infections pose a greater risk of spreading a disease further if not removed, since they can harbor pathogen spores.

"Sick" leaves that aren't producing infectious spores aren't risking more plant decline by being left as-is and are not necessarily a burden on the plant; in fact, the plant often sheds leaves with too much damage on its own anyway. While leaf yellowing can be triggered by multiple causes, it's often due to the plant itself cutting-off the leaf's supply of nutrients and water that causes it to yellow and fall off, not the disease within it. The process is similar to how trees in autumn pull resources like carbohydrates out of the leaf for storage elsewhere before cutting off the leaf's supply lines for water and sap and letting it die and fall off.

In your case, the holes pictured are minor and the leaf appears to be in good condition, so should be left alone and not cut off. That said, the plant can afford to lose a few leaves if necessary.

Leaf removal of any type, even healthy leaves, can trigger new leaf growth, because the plant is trying to supplement its ability to photosynthesize once enough tissues are lost. Therefore, you could cut back a plant without a pest or disease issue and one with a non-serious infection or pest outbreak, and both would be expected to leaf back out again. It's not something that should be done too often for healthy plants, since it uses-up stored root energy to regrow each time leaves are removed (and this type of gradual starvation technique by repeatedly removing leaves is one way weeds can be killed without using herbicide), but it is a useful tool to refresh the look of a plant that is succumbing to a temporary disease outbreak, like powdery mildew (not pictured in your photos, but another common affliction of peony).

Miri

You're welcome!