Contracts which involve part recovery which generate MKS parts cannot be failed or completed whether the part is destroyed or delivered to Kerbin. I would guess this is similar to a bug in vanilla KSP's contract which asks to add a part to a X vessel on a Y orbiting body with an engineer, the ID of the vessel you modify doesn't match the part ID in the contract, therefore impossible to complete.
However this said contract which involves part recovery which randomly generates vanilla parts can be completed.
Contracts which involve part recovery which generate MKS parts cannot be failed or completed whether the part is destroyed or delivered to Kerbin. I would guess this is similar to a bug in vanilla KSP's contract which asks to add a part to a X vessel on a Y orbiting body with an engineer, the ID of the vessel you modify doesn't match the part ID in the contract, therefore impossible to complete.
However this said contract which involves part recovery which randomly generates vanilla parts can be completed.