Jenkins 'Pipeline script from SCM' polls infinitely - jenkins-plugins

Question

My Jenkins pipeline workflow for building project is as below.
There are two files checked into repository -- JenkinsfileAllBranches and Jenkinsfile
1) JenkinsfileAllBranches - Polls all branches for changes
def scm_branch = ''
pipeline {
agent any
triggers {
pollSCM('* * * * *') //used this for quick debugging
}
stages {
stage ('SCM') {
steps {
script {
def git_scm = checkout([$class: 'GitSCM', branches: [[name: '**']],
doGenerateSubmoduleConfigurations: false, extensions: [],
submoduleCfg: [], userRemoteConfigs: [[url: <repository_url>]]])
scm_branch = git_scm.GIT_BRANCH.substring('origin\\'.length())
}
}
}
stage('Call Jenkinsfile for specific branch') {
steps {
print("branch:${scm_branch}")
build job:'Build_Project', parameters:
[[$class:'StringParameterValue', name: 'BRANCH', value: scm_branch]]
}
}
}
}
2) Jenkinsfile - For ease, I am providing the simplified Jenkinsfile
pipeline {
agent any
options {
disableConcurrentBuilds()
}
parameters {
string(name: 'BRANCH', defaultValue:'', description: 'Enter a branch name to build.')
}
stages {
stage ('SCM') {
steps {
script {
print("Parameter BRANCH: ${params.BRANCH}")
}
git url: <repo_url>, branch: params.BRANCH
}
}
}
}
Problem: First job is expected to poll every minute for changes in branches and second job is expected to build that specific branch where changes are found.
Everything works as expected when I leave the pipeline scripts in place.
But when I select 'Pipeline from SCM', JenkinsfileAllBranches behaves weirdly. It keeps polling the same branch again and again. How do I resolve this loop?
In the Stage -- Call Jenkinsfile for specific branch, I notice that it's always executing the branch as master instead of branch1 or branch2 where scm changes are found.