The Pipeline AWS plugin doesn't seem to recognize folder level credentials.  For example:

withAWS(region: "us-east-1", credentials: "my-aws-creds") {
    cfnUpdate(...)
}

If "my-aws-creds" are defined on the parent folder of the multi-branch pipeline job that runs this code from my Jenkinsfile, the creds are not be found.  If instead, I define them at the root level of Jenkins, it works.

It would be nice if folder level credentials were recognized, so administrators don't have to add credentials on behalf of developers.