Sculpt isn't the same as the version I saved

Does anyone know if this an issue with - or normal behavior for Blenders autosave?

Here’s what’s happening…

  • I’m working on a sculpt > Save AS a first incarnation.
  • Ruin it but run out of ctrl+z, so I want to go back to my saved version: File> open or go to recent.
    = saved version is the same as my ruined instead of my the one I saved.
    I have not saved manually in the meantime.
    If I re-launch Blender same issue, it’s happened to quite a few sculpts now…

Working in 2.90 but I can check up/down versions.
Is this just autosave overwriting the save AS file?
If so I’ve never seen it work that way, if you close the project without saving it should do so without writing any changes… right? Pretty sure I use that for modelling not infrequently.

Thank you!