The RHS say this:
Buddleja leaves affected by aphids show unusual puckering and discoloration, caused by aphids feeding on the undersides of the leaves. Over time, the damage curls the leaf and can create pale patches that look similar to disease symptoms caused by a virus. However, this leaf damage is likely to be caused by the melon-cotton aphid, Aphis gossypii.

The RHS are asking or folk to submit their findings on foliage damage to them.

I am on the South coast of England and my fairly new Buddleja is covered in damage from aphids!

As Shirley says, aphids
You may wish to read this blog on here which covers it, and also gives a link to RHS where you can report your plants

https://www.growsonyou.com/shirley_tulip/blog/36278-aphid-damage-on-buddleja