I had the booking done with Delta originally (on 006 ticket stock) for dates in July 2020. Routing was BUD-AMS-LHR overnight-SEA-PDX PDX-SEA stopover-LHR stopover-CDG-BUD. Most of the flights were predictably cancelled, when I called in to rebook for December they took my dates and changed the booking accordingly (routing was exactly the same).
However, at the same time, they changed ticket stock to KLM (074). Now I waited until one of the flights was cancelled and called Delta to cancel the booking entirely. Delta agents said they passed ownership of this ticket to KLM and are no longer responsible for it. KLM predictably said that from their p.o.v. the ticket was booked via Delta, and only Delta may refund me. To make things more fun, KLM said the ticket is not valid, likely because it wasn't properly reissued in July.
I submitted a refund request to Delta using old ticket numbers - their system conveniently doesn't accept the "reissued" ones because they are on KL stock. Waiting for their response but I am seriously considering to do a chargeback.