So I had this weird moment in an gen 7 Ag match where my lvl 1 magnemite got hit by a critical hit precipice blades from primal groundon and sturdy did not get triggered. It did not go to 1 hp instead it took 72%. I tried replicating it in a friendly but without succes. I am assuming it was a bug, but I have heard similar things happen in the real games. So I am really sorry if this is not relevant. I saved the battle so the link wil be below this, thanks for checking it out.
https://replay.pokemonshowdown.com/gen7anythinggoes-1166391156-o6i7hayqnf4wa1ip2ne31zec043hoabpw
I did a damage calc
252+ Atk Groudon-Primal Precipice Blades vs. Lvl 1 0 HP / 36 Def Magnemite on a critical hit: 8-64780 (72.7 - 588909%) -- guaranteed OHKO
Possible damage amounts: (64780, 8, 768, 1532, 2292, 3056, 3816, 4580, 5340, 6104, 6864, 7628, 8388, 9152, 9912, 10680)
Because of the modulo 65536, damage can become very small if it becomes a bit too large (for instance if you get 66000 damage, it becomes 464 (66000 - 65536) instead).
In this case, it can indeed do 8 damage and not ohko Magnemite (I don't know if these were the precise spreads of the involved Pokémon but it shows it is possible)
Also this calc shows something else : it shows guaranteed OHKO because the first value of the damage range is high enough to KO Magnemite but the minimum value isn't in first place because of the modulo 65536.