What recently happened with me is that we proved an open problem, and while writing it up, we found that the result was claimed in an arxiv paper that never got published anywhere. Their proof was incomplete.
This meant that we had to acknowledge that the result already was claimed by someone else, but that we couldn't understand the proof attempt.
This of course leads to problems because now our result is somehow viewed as less valuable, and more difficult to publish. Which again means that research in that area will suffer from an unpublished claim.
What are you measuring to determine that now your result is viewed as less valuable? Is it the peer reviewers or personal conviction? In the end all that matters for you, to the extent that peer-reviewed publications are important to your career, is that your paper gets cited. And if people are citing the unfinished arxiv paper instead of yours then that speaks poorly of their own research and not yours.
This meant that we had to acknowledge that the result already was claimed by someone else, but that we couldn't understand the proof attempt.
This of course leads to problems because now our result is somehow viewed as less valuable, and more difficult to publish. Which again means that research in that area will suffer from an unpublished claim.