What is the type of the next schema item ?
I am surprised the credential is changed back, it used to stay there. Either it is a bug or you do something I did not experience.
I may have another work around for you.
- Create an attribute of type credential, set ti to your system account
- put everything that need to be run as the system account in a separate workflow.
- From your main workflow, call this workflow as a nested workflow, click on this nested workflow, then on connection info. Then use Dynamic, select your credential.
This should allow to run the nested workflow as a separate user.