Copyright is a tangled subject, but the physical recording would typically be the property of the record company, who would of course expect a mixed master via A&R or the album producer, but would typically have legal ownership of the multi and commonly take custody of it. Safety copies would be made of masters but not, to my knowledge, of multis. Some canny artists & management ensured ownership of such tapes either immediately or subsequently. My understanding is that rights in masters & multis can revert to artists once outside contract, but I think that’s essentially a matter of retrospectively untangling legalities that were given little thought at the time (thinking 10, 20, 30 years ahead has tended not to be a strong point for musicians or record companies).
It is astonishing that the multis of what has come to be seen as such a classic album were treated so cavalierly, but given the triangulation between artists, management and record companies it is not hard to imagine assumptions and staff changes leading to things falling between the cracks and the problem only being discovered long after the fact. In the case of Trident there were rumours of things being looted rather than binned: I'd be interested to know whether the interested parties have tried dropping a few grand on a private investigator.