An user have only access to tasks and can't see the sub-tasks. When he export all fields to excel the Sub-Tasks ID are shown

[JoséF]

I had use the solution mentioned by @Vidic Florjan on this link https://answers.atlassian.com/questions/230835

Can this be consider a bug?  

[Nic Brough -Adaptavist-]

It's not a bug.  The point of JIRA is tracking stuff, not obscuring things from users.  The structure to hide bits of a task (i.e. it's subtasks) simply isn't there.  Florjan's workaround is clever, but it's not going to work everywhere because JIRA doesn't want to hide this data - it's very nature is to show the users all the information.  I don't think you can work around that design.  Still, at least the users can't get the subtask details.