This seems similar to JENKINS-67857, which was caused by the SECURITY-2441 or SECURITY-2463 fixes in Jenkins Security Advisory 2022-02-15.

Which type of project are you using? What setting in the project causes A.groovy to be loaded?

This issue has component = groovy-plugin and labels = pipeline workspace, but the Groovy plugin documentation says "Currently the plugin does not support pipeline syntax", so it's not clear to me whether the issue affects pipeline projects or freestyle projects.

Assigned to the workflow-cps plugin because that is the plugin that implements the load step.

Sorry if I've set wrong labels.
I've attached screenshot in the description with the place where I am loading the main A.groovy file in jenkins. And additional screenshot with the folders structure. So, when the project is being built, there is no problems if I create the next code in the `fastfile`, because there is no unrecognisable additional subfolder inside the Job Name folder unlike the Job Name@script folder.

slather(
 cobertura_xml: true,
 proj: "Project Name.xcodeproj", //relative path, file found correctly
 output_directory: "./test_output", //relative path, file is created in correct place
 scheme: scheme,
 jenkins: true
)

Are A.groovy and B.groovy in the branch that is checked out to "workspace/Job Name"?

If they are, then B.groovy should get checked out to "workspace/Job Name/jenkins/B.groovy" as well, and load(env.WORKSPACE + '/jenkins/B.groovy') should find it from there.

If not, then perhaps you can use readTrusted to read 'jenkins/B.groovy' from SCM, writeFile to write that to a known path in the workspace, and finally load from there.

Here is what I use to find the B.groovy file (sure, these files are in same repo and even same folder), but don't want to use that unnatural way.

git_jenkins_folder = sh (
        script: "find \"" + WORKSPACE + "\"@script -type d -name 'jenkins'",
        returnStdout: true
    ).trim()

utils = load("$git_jenkins_folder/B.groovy")

You say `If they are, then B.groovy should get checked out to "workspace/Job Name/jenkins/B.groovy" as well`. No, as I mentioned in my issue, they both (whole project actually) are checked into
/Users/user/.jenkins/workspace/JobName@script/ecb7a9317b1ad672698830264d9e0ce2b9b6f330c043bb85f48623f3cdcab65e/jenkins/A.groovy

and

/Users/user/.jenkins/workspace/JobName@script/ecb7a9317b1ad672698830264d9e0ce2b9b6f330c043bb85f48623f3cdcab65e/jenkins/B.groovy

of course as these 2 are in same repo and folder. I am getting that extra ecb7a9317b1ad672698830264d9e0ce2b9b6f330c043bb85f48623f3cdcab65e subfolder as I've mentioned in the description on the 4th row.
Project to build checked into workspace/Job Name/jenkins but later.
First script is checked into workspace/Job Name@script/jenkins but with extra folder so we get workspace/Job Name@script/jenkins/ecb7a9317b1ad672698830264d9e0ce2b9b6f330c043bb85f48623f3cdcab65e

As designed I think. The load step is for loading a file in an agent workspace, not on the controller.