It seems that "withMaven" is not picking global configuration correctly when maven is defined under "tools" although logs indicate it otherwise. 

Here is sample pipeline:

pipeline {
    agent any
    tools {
        maven 'maven3'       //1
        jdk 'openjdk-11'
    }
    stages {
        stage('Not Ok') { 
            steps {
            	withMaven{       //2
        	    sh "mvn help:effective-settings"
        	}
            }
        }
        stage('Ok') { 
            steps {
            	withMaven(maven: 'maven3'){        //3
        	    sh "mvn help:effective-settings"
        	}
            }
        }
    }
}

I have defined my Global Settings settings.xml (under Managed Files) and it's configured under Global Tool Configuration.

When you run the pipeline then at (2) in logs this can be found:

[withMaven] using Maven global settings.xml '4a3dd40e-13fa-4d56-9a7d-6c2390e85aa2'  

but maven effective settings does not print my settings.xml.

However, when I explicitely define maven with (3)

withMaven(maven: 'maven3')

then effective settings include my settings.xml.

Please check attached file.

I would expect that it is enough do define maven once (1) to pick correct settings.xml.