In my scripted multibranch pipeline (using GitLab branch source) I use something like

node(label: 'builder') {
  checkout scm
  ...
}
node(label: 'test') {
  dir('src') {
    checkout scm
  }
  ...
}

Sometimes, the second checkout does not work because

[GitCheckoutListener] SCM 'hudson.scm.NullSCM' is not of type GitSCM

No error is thrown, tough. The build continues and fails later because there are no files in "src". This happens only very occasionally, simply re-running the pipeline will work in most cases.

Any clue how "scm" can become a NullSCM during the build?