For your issue encountered, I believe this is due to the Scrum Board by design does not rolled up the estimates on the subtask as this is not appropriate on the Agile standpoint. From an Agile standpoint, sub-tasks should not have to warrant story point as those point should roll up to the parent issue and should be accounted for there. If they need story points of their own, they would be spun off into a full on Story of their own. They would then be counted in the burndown charts as well in that manner.
And furthermore, for Agile, stories are actually considered as very similarly to a "subtask" for thier parent issues which is Epics. It is recommended to group the stories into an Epic which I see that you have started to utilize this feature as well.
Hope this answers :) Cheers
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