I believe the 407.54MB is size of the local file that has your data. I wonder if there is a discrepancy between the 500MB data limit in QuickSight and the local file size. Basically, QS has reached its 500MB limit but the resulting file size is less than 500MB.
@Sanjeeb2022 or @Koushik_Muthanna thoughts?
Edit: Ticket will be raised by OP in order for the QuickSight Team to take a closer look.