Board Thread:Game Discussion/@comment-3554435-20180330072650/@comment-2001:16B8:6792:1D00:3C27:88C1:B46:A2DE-20180330095216

Source is their homepage. Last sentence: New FLASHBACK Events:

Flashback events have returned for 2018, and are available to start at any time during the update. Once an event has begun, however, no other Flashback event may commence until the active one has concluded.

Flashback events now have an updated reward system. Players on their first playthrough of a flashback event will receive the original special event rewards. Players repeating a flashback event can claim stage rewards for any stage they had not previously completed and will receive a final R$/Gold reward regardless