This is not a bug. The Imps have 2 damage reduction.
ah, nice catch ! Thank you Kampfrosch94 : very few would have noticed. I thought it was a possibility, but underestimated the little buggers.
nb : "Damage Resistances Cold; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Silvered."
[ which is half damage ]
https://www.dndbeyond.com/monsters/impCheers