jenkins pipeline when is evaluating to false though file exits

Question

any hint why WHEN expression is evaluating to FALSE ? I tried this solution but no luck

    stage('Code Coverage Tests') {
        steps {
            container('main') {
                catchError(buildResult: 'SUCCESS', stageResult: 'FAILURE') {
                    sh '''
                    pwd
                    echo testinfo > /coverage-data/_coverage_report.dat
                    ls -ltr /coverage-data/
                    '''.stripIndent()
                }
            }
        }
    }
    stage('Code Coverage Scan') {
        when { expression { return fileExists ('/coverage-data/_coverage_report.dat') } }
        steps {

output shows file exits.

+ ls -ltr /coverage-data/
total 4
-rw-r--r-- 1 jenkins jenkins 9 Dec 18 18:13 _coverage_report.dat
+ pwd
[Pipeline] stage
[Pipeline] { (Code Coverage Scan)
[Pipeline] fileExists
Stage "Code Coverage Scan" skipped due to when conditional