'Show parent issue...' scripted field isn't indexed at time of child issue creation

To reproduce:

• Set up a 'Show parent issue in hierarchy' scripted field (call it 'parent issue field') with Target Issue Type being 'Story' and with 'Subtask Parent' as the only parent navigator.
• Find or create a Story issue (call it ISSUE-123) and create a subtask under it.
• Do a JQL search for "parent issue field" = ISSUE-123.

Expected: the new subtask appears in the results.

Actual: the subtask doesn't appear.

This is probably because the subtask gets indexed after it has been created, but before the parent-subtask link has been set up. Therefore the scripted field, when called, cannot find the parent issue and so returns a null value.