Certified GKH Gardening Expert
This is an early fungal issue in the soil. It can be corrected easily at this stage.
Using Dolomitic lime and wettable sulfur together will help balance out the pH of the soil along with killing off any bad microbes lurking around, making it a suitable environment for the plant, and not the disease.
Here is a link that will give you more information on the care of these shrubs. This link is for the variegated type, but it has the same care: https://www.gardeningknowhow.com/ornamental/shrubs/golden-euonymus/golden-euonymus-care.htm