In a workflows containing a sub flow, variables are added by the workflow admin or any admin that can add sub flow variables, to the subflow.
Once the variables are added, they are not able to manually edit them.
Please make sure that
sys_security_acl_a5c1f6033b021000dada82c09ccf3de4.xml is on your instance as this acl needed to allow editing access for subflow variables.
It masks the high level *.* edit ACL.
Another solution is to edit the high level *.* ACL, this idea was also a suggestion by Development