Metallicity standard deviation vs. age trend of the stars under investigation (blue line for- main trend; blue region for measurement uncertainty Sect. 3.1). Orange: GES accretion event (observations: Ruiz-Lara et al. (2020); Gondoin (2023); Lu et al. (2024); Ratcliffe et al. (2023); Anders et al. (2023), numerical simulations: Laporte et al. (2019); Belokurov et al. (2018); Helmi et al. (2018); Buck et al. (2023); Wang et al. (2024)); red: Sgr (Ruiz-Lara et al. 2020); gray: mergers of the Milky Way dwarf galaxies (Wang et al. 2024). The solid vertical lines show the beginning of the merger, and the dashed vertical lines show the peak of star formation corresponding to a particular accretion event.