This issue was originally created in Sleuth: spring-cloud/spring-cloud-sleuth#1630
I investigated it a little bit, you can see my findings here: spring-cloud/spring-cloud-sleuth#1630 (comment)

Description:
It seems if the system class loader is wrapped (e.g.: spring-boot-devtools restarts the app using a RestartClassLoader), cglib produces incorrect ClassLoaderData causing proxied package-private methods returning null (public/protected is fine); please see the original issue and investigation details.

Here's a sample project (uses 2.3.6.RELEASE) that reproduces the issue: https://github.com/jonatan-ivanov/sleuth-gh-1630-minimal Please check the readme, the issue is hiding.