Configuring Jira faced with the fact that I can not give access to the system user to a separate Issue in a particular project.
Of course, there is a way out - these are Security Level settings, but, unfortunately, this method did not suit me, although it is the basis of the invented setting option. The method of trial and error greatly develops cognitive abilities and firmly reinforces the acquired skills, but takes a lot of time, so I share what happened with me and invite you to the discussion. I hope this will be useful and will save some time for Jira administrators.
The problem of setting up through the Security Level is that access to the Issue is provided to all users of a specific Group / Role, and it was necessary for me that only certain users in the Group / Role "A" see some of the Issue project. To add individual users to an overall Security Level Scheme task is an overwhelming task for an ordinary user of the system, and an employee with administrative rights is always busy. Moving from verbiage to the recipe.
We have Atlassian JIRA v 6.3.8
Server Instance .
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Free. Allows you to add Watchers at the time of Issue creation, for this you need to add Custom Field "Watchers" to the used Screen Schemes. This add-on will also allow the use of Custom Field in the Security Level setting.

2. Configure Permission Scheme
In the Permission Scheme of the project, add the necessary group or role in the Project Permissions, Browse Projects rule. It is worth noting that when adding a group, all participants will see project āPā in the list of projects, but without Issue. When adding a Role, only those who have been added to the Role in the project settings will see the project. A specific project can be administered by far more users than by administering the entire Jira, and without harming the integrity of the system. Then you can enter the Group / Role in a few more rules. For example, Create Issues will allow users to create an Issue within the project and see only them (you need to add a Reporter to the Security Level), plus those in which they are Watchers, you can also allow them to leave comments, edit them, add attachments, etc.
3. Configure Security Level Scheme
Add a new Security Level Scheme and create a Security Level in it, in which we add all Groups / Roles, which should always see all the Issue of the project,
DO NOT add Groups / Roles, which only certain Issue should see. And add:
User Custom Field Value (Watchers) - allows the added user in Watchers to see the Issue;
Reporter - the user will only see the Issue created by him;
Current Assignee (optional) - allows you to open access to the Issue to a user from Group / Role "A" without adding it to Watchers, simply by assigning Issue to it.

As a result of such not tricky manipulations, you can give access to a specific Issue to a specific user, both at the time of Issue creation and later, by editing Watchers or Assignee.