Oh no! Your neighbor's bush is actually serving as a nearby place for them to converge to breed. I would assume that they aren't doing anything to control them which is bad news for you. You might talk to your neighbor about letting you use an insecticidal soap spray like once a week on their butterfly bush while the beetles are active. If you can keep the population down on the butterfly bush, it may help contain them to the butterfly bush so they aren't spreading to your garden as bad. Might be able to find a recipe for a cheap homemade version of a spray that would help to keep the population at a more reasonable level. The Old Farmer's Almanac has a recipe for one here. Basically just vegetable oil, alcohol, dishwashing detergent (may want to check up on this, pretty sure most of the recipes I've seen say not to use detergent to use dishwashing soap, like Dawn), and water. Just be sure to do a small test patch before spraying the whole plant so you don't end up killing your neighbor's bush.