This was previously working using the following calculation but now it seems to have stopped working properly for some reason 
{{#=}}{{issue.subtasks.original estimate.sum}} / 3600{{/}}
Here's the entire rule:
Original Estimate changed on sub-task → update total estimate on parent

For reference: the Issue Types used above are all sub-tasks.

The rule is actioned as a SUCCESS in the logs and so here's how it looks on the ticket's History:
