What incentive would LH have to provide valid vouchers, which can be redeemed? None, really!
Much easier to ask the passenger to handover fresh cash for a new ticket.

As I said even last March - I consider these vouchers to be a scam. You don't get them, you cannot redeem them, you cannot apply them for all routes (it has to be the same country origin), you are not allowed to use them for domestic routes, you cannot book through all sales channels, you are at the mercy of the revenue management system, which may identify and inflate the price of the flight ..... etc., etc.