According to Configuration JIRA option - Advanced Document, the modification should take effect immediately. "jira.table.cols.subtasks" doesn't take effect after modified.

Steps to replicate

1. Change "jira.table.cols.subtasks" setting via Administration>General configuration>Advanced Setting.
2. Go back to issue contain subtask, the modification didn't take effect.
3. Restart JIRA doens't helps.

Workaround

1. Create jira-config.properties ( if doesn't exist)
2. Manually add the parameter like below:

   jira.table.cols.subtasks = assignee, progress, status,priority, issuetype
   

3. Restart JIRA to take effect.