Summary Field is showing blank when there is data to return

You can allow the Details table to bring in lookup fields from both parents along with any hard data for the Details record you want to input. The Details records can be strictly lookup fields too if you wish. You can then create your summary max date field in any one of those two parents to provide the max create date or any date field you wish to summarize from the Details table if that is still an item after you reconstruct the relationships.