Zoo managements increasingly do not cherish the individual tigers in their care. The Breeding Programme is seen as the be all and end all for holding tigers, which is not surprising as tiger cubs are revenue generators, and some UK Zoo managements would want to dispose of non-productive animals in the same way as Danish zoos, but the public 'wouldn't understand'.
The Sumatran pair at Aalborg in Denmark, Batu & Kim, were supposedly retired but produced 2 cubs before Kim was euthanised: Mai-Mai died of strangulation aged 7 months and Khai was euthanised at age 2 as his genes were over-represented in the breeding programme, which would have been known even before he was born. And there has been no further news on the 2 male cubs born to Batu & Bethari in April 2019. At Ree Park Asim & Gasha had 4 litters. A male cub from the first litter was euthanised because he had a white tail and white paws (and therefore not regarded as 'pure' Sumatran); their 3rd litter of 3 cubs born in Sep 2018 are no longer heard of after Oct 2018. The 'plan' to swap Asim and Jae Jae would have been made before this litter were conceived and had Jae Jae arrived the Breeding Programme would've wanted him and Gasha to form a breeding pair almost immediately, as was the case with Asim & Melati at London Zoo (but that didn't go to plan either).
Euthanising tigers that are no longer actively involved in the Breeding Programme, being too old or 'over-represented in the gene pool', could happen at UK zoos. Such a situation would probably be reported as the result of 'age-related issues', understandable in an elderly pair, and would not raise suspicions if the two were euthanised a couple of months apart rather than simultaneously (particularly if new tigers arrive to replace them very shortly thereafter).