Simple Jenkinsfile:

if (manager.setBuildNumber(1)) {
  echo "Current build: ${manager.build.number}"
  manager.createSummary("gear2.gif").appendText("<h2>Testing</h2>", false)
}

Output:

[Pipeline] echo
Current build: 3
[Pipeline] End of Pipeline

3 is the current build number so this number changes on every build and the createSummary() is added to the current build instead of build number 1 (not what I expect).

Expected behaviour is to see build number 1 being set and the summary being added to build number 1.