The calculation currently only sums columns from the main tasks. Since timetracking column is also available on subtasks, it should ideally sum these as well.
Having the same problem and havent find a anwer yet. I really need the subtask functionallity and the dashboard not showing the time in this type of task is killing me.
Hi all, I’ve put together a workaround in my board for this problem and it seems to have done the trick. Here it is:

In the subitems, open the time tracking column settings and select “Show Summary on Parent Item”. I renamed the column ‘Subitem Time Tracking’.

For the time tracking column on the parent menu, I renamed it ‘Parent Item Time Tracking’.

Now add a Formula column. I renamed it ‘Time Totals’. Enter this formula to add the two columns together and equate to total hours:
SUM({Subitem Time Tracking},{Parent Item Time Tracking})/60/60
 Now SUM will appear in the bottom right hand corner of the item. Click on SUM and you can add your own UNIT title  I added Hours. So now at the bottom of the task I can see the total hours, including subitem hours!