I managed to reproduce the problem locally and reliably using these commands:
mvn clean install -DskipTests mvn docker:start -Pci-mssql -pl build/parents/integrationtest -Dtest.database.run.mssql.skip=false i=0; while mvn verify -f integrationtest/mapper/orm-coordination-outbox-polling -Dit.test='Outbox*Concurrency*' -Dtest.database.run.skip=true -P ci-mssql -Dtest.launcher.args='-XX:ActiveProcessorCount=2' 1>/dev/null 2>&1 ; do (( i ++ )); echo "Succeeded; trying again ($i)..."; done ; echo 'Failed!'
It generally takes less than 4 attempts to reproduce the problem on my machine.