This Jira seems to imply that it does: [SONAR-11853] Auto-configuration pull requests on Jenkins - SonarSource
But as you suggested, I changed the Jenkinsfile to this, and it works (just messy).
def changeUrl = env.GIT_URL.split("/")
def org = changeUrl[3]
def repo = changeUrl[4].substring(0, changeUrl[4].length() - 4)
if (env.CHANGE_ID != null) {
sh "mvn -B sonar:sonar \
-Dsonar.projectKey=${org}_${repo} \
-Dsonar.pullrequest.provider=GitHub \
-Dsonar.pullrequest.github.repository=${org}/${repo} \
-Dsonar.pullrequest.key=${env.CHANGE_ID} \
-Dsonar.pullrequest.branch=${env.CHANGE_BRANCH} \
-Dsonar.pullrequest.base=${env.CHANGE_TARGET}"
} else {
sh "mvn -B sonar:sonar \
-Dsonar.projectKey=${org}_${repo} \
-Dsonar.branch.name=${env.BRANCH_NAME}"
}