Similar to JENKINS-34416 (and might be another bad examples interpretation), having several global classes, an invocation from the Jenkinsfile makes a different object than a invocation from another class.

Running a job with https://github.com/witokondoria/jenkins-multibranch-test as implicitly loaded global library and running the same repo Jenkinsfile, the job console output shows two different objects for invocations from Jenkinsfile:7 and closure vars.flow:24

Might be related to this, action.otherAction() cant be invoked from within the Jenkinsfile (replacing Jenkinsfile:7)