Most literature raves about the lack of diseases that buddleia will get, so it was difficult to find good research-based information on this.  https://www.gardenguides.com/98594-butterfly-bush-diseases.html  The main diseases they mention are rhizoctonia and phytophthora, both of which are difficult to treat effectively as they are soil-based.  You said the roots look in good shape anyway... One way you can test for root rots is to take a root about the size of spaghetti and see if you can slip the outer root cover off the inner cover.  If it slips off, then it's a root rot.  The stems appear to have some brown areas too, almost like a stem canker, but I couldn't find any information about this on buddleia.  

Have there been any herbicides sprayed in the area?  The cupping and twisting of foliage look like either a virus (again, could find nothing in the literature on this being an issue) or herbicide damage. 

Of course, we always have to consider the soil and what is in it. Even reputable soil distributors have had problems with excess salts or even herbicide residues in their soils.  If the application of new soil coincided with the problems with your plants, I would want to look at this more closely.  You can get a soil test done for free from the NCDA Soil Testing Lab (Agronomic Division) in Raleigh. https://www.ncagr.gov/agronomi/uyrst.htm  I would recommend doing this, but it will only tell you the pH and the fertility levels. However, if something is off there, it could cause tissue burn.  To test for herbicide contamination, you can plant a small tomato seedling, and if its leaves start to grow in a deformed pattern, there may be residual herbicide in the soil. Tomatoes (and maybe buddleia) are very sensitive to trace amounts of herbicide found in some composts. 

You don't say what kind of mulch you have, but it would definitely be good to get your soil tested to see what's in it!  

Check out this publication https://content.ces.ncsu.edu/butterflies-in-your-backyard