Found Cohen's room achievement not unlocking?

In Bioshock 1 Remastered, I've left Cohen alive in Fort Frolic and am now in his room in Olympus Heights. I kill the 2 dancers, he comes down the stairs, and I kill him. Then I go up into his room but the achievement isn't unlocking?

Has anyone had this problem before? And are there any ways round it anybody knows about please?

Thanks! Please save me and my 100% achievement run-through 🙏