Why are Folder Properties key/value pairs not recognized in multi-branch pipeline job?

[ZillaYT]

I installed the Folder Properties plug in, and define these

GIT_BRANC = feature/dev-1234-some-work

RUN_TESTS = true

DEPLOY_TO_SANDBOX = true

I then have this in my Jenkinsfile

def echoStr = """

****************************************************************************

Git branch: ${env.GIT_BRANCH}, Run tests: ${env.RUN_TESTS}, Deploy to sandbox: ${env.DEPLOY_TO_SANDBOX}

****************************************************************************

"""

println echoStr

and I get this in the console

****************************************************************************
Git branch: null, Run tests: null, Deploy to sandbox: null
****************************************************************************

Why is that?

Chris