Spotted Lanternfly feeding is minor on most garden plants, including Milkweed as far as we're aware, and this symptom was not caused by their activities. Instead, it's very common for the oldest, lower leaves on a variety of perennials (and even shrubs and trees) to turn yellow and fall off by midsummer or so. The plant, if otherwise growing normally, has enough foliage to sustain itself even with some leaves lost, and they jettison those that are not getting as much light on the stem bottoms, since they're the oldest foliage and now of less use to the plant. Plants stressed by under- or over-watering, or too few nutrients (more of an issue in pots than plants grown in the ground), can exacerbate this lower-leaf yellowing, but here too, if growth and flowering has been normal, this is not a concern.

Miri