Certified GKH Gardening Expert
It appears to be an infection, whether fungal or bacterial. It may just be that it is not as hardy to the infection as the other two.
If a fungicide does not correct the issue, then it may need to be replaced with another healthy specimen.
Here are some articles that will help:
https://www.gardeningknowhow.com/ornamental/shrubs/potato-bush/potato-bush-information.htm
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/plant-problems/disease/using-lime-sulfur-in-gardens.htm