When 'Visible on Summaries' is set to false for a variable, after submitting the item, the variable is visible in details section on ticket page
Any supported version
There was a known problem for this issue in earlier versions which is fixed from Jakarta release. The most probable cause for this is using a customized version of 'Ticket Fields' widget cloned from Pre-Jakarta .
Revert to OOB 'Ticket Fields' widget
Documentation link for information on how 'Visible on Summaries' works.