What version of Hugo are you using (hugo version)?

$ hugo version
Hugo Static Site Generator v0.65.3/extended linux/amd64 BuildDate: unknown

Does this issue reproduce with the latest release?

Yes

I'm a big fan of the post_slug_or_ref/index.md file structure because it makes it easy to keep track of post-specific assets. Hugo 0.65.0 broke {{< ref other_post >}} cross-linking between any two pages that use this directory-based structure, with the following error:

ERROR 2020/02/24 20:46:56 [en] REF_NOT_FOUND: Ref "some-other-page": "/.../hugoBasicExample/content/post/some-page/index.md:1:22": page not found

Here's a sample repository that demonstrates the issue.