While it is not possible to give a definite diagnosis from a picture, it appears that you may have damage from four-lined plant bug. These common insects feed on plant leaves and leave the round dark spots. Members of the mint family are a favorite of four-lined plant bug, but there are over 250 other susceptible species including butterfly bush. The sunken black spots are characteristic of the insect damage. Having heavy damage on some leaves with others even on the same stem unaffected is also indicative of this insect. While the damage to the leaves cannot be reversed, there are some control measures that can be taken. However, unless the damage is extensive there is not often anything that needs to be done which is why your bush can still be growing and blooming fine. I am including a couple of links to bulletins with more information about the insect with identification pictures. You may or may not be able to find the actual insects. If you find insects and wish to do something about them the bulletins also give information about the lifecycle and control measures.

https://www.canr.msu.edu/news/leaf-spot-damage-fourlined-plant-bug-michigan-gardens

https://entnemdept.ufl.edu/creatures/veg/leaf/Poecilocapsus_lineatus.htm#:~:text=Fourlined%20plant%20bugs%20overwinter%20as%20eggs.&text=Over%20the%20next%2017%2D20,of%20the%20current%20season's%20growth.