[JENKINS-39592] Unable to use global function defined within library project - Jenkins Jira

Defining a shared library with this structure:

 +- src
 |   +- org
 |       +- foo
 |           +- Bar.groovy  # for org.foo.Bar class
 +- vars
 |   +- execEcho.groovy

Where Bar.groovy is:

class Bar {
    def steps

    Bar(steps) { this.steps = steps)

    def doEcho(message) { steps.execEcho message }
}

and execEcho.groovy is

def call(message) {
  echo message
}

Using this code within a pipeline

...
def bar = new Bar(steps)
bar.doEcho("Hello World!")
...

Fails with:

java.lang.NoSuchMethodError: No such DSL method 'execEcho' found among steps [...] or symbols [..., execEcho, ...]

Even though execEcho shows up as one of the symbols