They do backfill, sometimes. However, they haven't yet. Depending on what chart SW you are using you may need to take action to get it to backfill.
Sierra won't backfill if there is valid data at the end of the file. So you need to select Edit Minute Data from the right click menu and then Delete to end of file. You need to move the chart so the point you want to delete from is the right most point visible first. That will then delete the data and MT should backfill.
However, I've just done that and it only backfilled from 8:47 again, so clearly the earlier data is still missing at present.