This change was announced six weeks ago... and they even delayed it by a month.
"In order to help prevent the exploits that the 7-day cooldown period was meant to address, the season reward loot chests will be changed to be based on each player's current league at the end of the season rather than their highest league achieved during the season. This change will take effect for the current ranked play season which ends on August 15th."
https://peakd.com/splinterlands/@splinterlands/sps-staking-and-other-splinterlands-updates