Rollback failed: serviceaccount not found

After upgrading the chart from 2.9.4 to 3.2.0, everthing seems to work fine. However, in order to check if a rollback would work, I have tried to rollback from chart 3.2.0 to 2.9.4 and the error I got is:
**failed error: pods "pulsar-zookeeper-2" is forbidden: error looking up service account pulsar2/pulsar-zookeeper: serviceaccount "pulsar-zookeeper" not found**

I dont understand why it is looking for a service account which in version 2.9.4 should not be present. Indeed from the command 'helm template' with version 2.9.4, there is no reference to this service account. But when I check for the deployed statefulset there it is:
serviceAccountName: pulsar-zookeeper

What is the reason and how can I solve the issue?