Based on a YouTube guide video for the 2021 Spring Festival, it looks like ‘Shrine Breaker’ is supposed to start automatically after completion of ‘Shrine Raider’. On my main and my alternate the quest line stops after ‘Shrine Raider’, there is a blue diamond above the NPC’s head outside Pier’s shop but no Spring Festival quests in my book.
I play on quest(2) link, so I tried standalone with no change. I’ve visited the NPCs that are involved and waved at them with no response, logged, and submitted an in game bug report.