@Samskara I tried deleting it from the CLI, first but it failed. I don’t remember the exact message anymore.

Then I tried deleting the same image using @franzbroetchen 's method and it worked, even if was the same snapshot.

@franzbroetchen You’re my new hero, I’ve search everywhere and never found that tip. I had 230Gb worth of broken APFS snapshots, that are now gone!