Hi Julie,

This looks like chlorosis. See these pages:

https://hort.extension.wisc.edu/articles/chlorosis/

https://extension.illinois.edu/plant-problems/chlorosis

This issue can be linked to conditions like excessive or too little water and heavy soil. Sometime the condition comes and goes. Unless it is persistent, it rarely kills plants.

FYI, burning bush (Euonymus alatus)  has been classified restricted noxious weed in Minnesota because it has begun to self-seed in natural areas. Plants can no longer be sold or transported here.

I hope this helps! Thanks.