(Sub-tasks must be enabled in JIRA)

When creating a new filter from Simple mode, if you select the "Standard Issue Types" parent option for the issue type field in the filter form, then when you switch to Advanced mode, the generated clause will be type in standardIssueTypes(). You can then modify this query in Advanced mode (e.g. add AND status = Open or something that fits), then switch back to Simple mode, and the "Standard Issue Types" parent option will still be used.

If you create a new filter from Advanced mode however, and begin with the clause type in standardIssueTypes(), then when you switch to Simple mode, instead of the "Standard Issue Types" option being selected, it is actually the explicit list of all standard issue types multi selected. Thus, if you modify the filter, then go back to Advanced mode, you no longer have the original clause but something like type IN (1, 2, 3, 4) instead.