Why does the security level keep returning to "Viewable by all users" when set to "Manager"

[Jay Stone]

I am classed as a manager and created a number of issues and assigned them to me. 

When I subsequently change the security level to manager only it saves (I get a pop up) but then when you open it in JIRA it has reverted back to the previous viable by all users.

I tried editing the issue and doing bulk change, but behave the same way. 

I need to lock out viewing access to managers due to confidential information within.