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I think this is already covered in Mass Monster-Slaughter Sidequest or 20 Bear Asses - i.e., the description on "Mass Monster Slaughter Sidequest" specifically states "Why any monsters of these types didn't count as kills before the quest is rarely addressed..."
Not quite, since it's more than just killing enemies.
For instance, Burrito Bison has only three missions at a time (such as "make X cash", "travel X distance", "kill X enemies"), and completing one lets a new one take its place. Realm Grinder has different mechanics be available at different milestones and unlocking them only takes stats from after the milestone is eached (so if one requires a 2-hour game session, it'll start counting from that milestone, ignoring an earlier 8-hour session).
A video game mechanic where a mission, achievement or perk requires accomplishing a task with certain restrictions that are only counted from the moment you learn of it, ignoring the prior occasions where it was done. Common in Idle Games.
e.g. An quest requiring the player to kill twenty enemies with a fireball that doesn't take into account that the player once killed twenty-six enemies with a fireball, long before reaching the questgiver.
Edited by Chabal2