According to Investopedia, "in May 2020, the definition of M1 changed to include savings accounts given the increased liquidity of such accounts."

If you look at M2, which already added these in, the increase looks much smaller.[1]

[0]https://www.investopedia.com/terms/m/m1.asp

[1]https://fred.stlouisfed.org/series/M2NS

Seems like this is probably it:

https://fredblog.stlouisfed.org/2021/05/savings-are-now-more...