I had booked a flight for myself, and a second set for my son/wife (a on other dates), and later asked LH to split their booking so that my wife could join me on my flight back (while my son had to leave on another date)
We had already selected (and paid for) seats for the on their original booking, so when called the customer service
to make seat arrangements for my wife on our common return flight, I had to pay for the new seats gain, and LH told me that the payment for the original seats was non-refundable?

Note that both I and my wife are *G via United, but that doesn't seem to count for anything (neither the seat costs nor type) on LH these days. None of the few service agents I talked to even started to entertain the idea that being *G gives you access to better seat, or reduced the price of the seats.

Is it really the case that when changing a booking you completely forfeit the extra payment you made for seats on the original booking?

Lufthansa seems to be hard up to need to scam passengers like that

Is there a better customer service contact I can bring this up with?