Something happened within the update of Github branch source plugin 2.3.0 update when combined with the update to Jenkins core 2.73.x where when you have a branch with a # character in the name it will not be able to find the branch name.

Steps to recreate:

1. Create a shared library with a branch name "Branch#Test"
2. Attach that shared library to a Jenkins instance as a global library using the github modern SCM
3. Create a job which loads the branch by doing @Library(SharedLibrary@Branch#Test)
4. observe the job fails with 
   No version Branch#Test found for library GithubLibrary

This issue only happens when you update the github branch source plugin to newer than 2.3.0 and have a current version of core(from my testing) which means something in the 2.3.0 release changed this behavior.

Let me know if I can provide more information