My understanding is that vouchers weren’t previously available for part-flown itineraries but they are now.
its probably a case of your cancelled segment/itinerary showing as a voucher on the system which is your 125 ticket number although you never received one.
I assume they have asked the fares team to calculate the value of your part-flown booking. This will take some time, due to the back log.
however, given you received that email advising you a voucher is not possible, I would escalate and maybe send a DM on Twitter as your flight was cancelled.