If it was placed in the world before the thrall stat update, they didn’t get a Grit stat because they didn’t have one. Or rather, they got a 0 stat, and a nominal 40 % increase chance that is actually a 0 % chance, but the interface doesn’t know numbers lower than 40. Probably half of my T4 thralls on Siptah predate the Grit stat, and thus have 0 Grit at level 20.