This is not a new issue; behavior present in v0.54.0 through v0.100.2.

config.toml

baseURL = 'https://example.org/foo/'

template

{{ $url := "/foo/bar" }}
{{ $url | relURL }}
{{ $url | absURL }}

result

/foo/foo/bar
http://localhost:1313/foo/bar

Notice that the subdirectory from baseURL is repeated when using relURL, but it is not repeated when using absURL.

This has come up a few times in the forum, usually in conjunction with menu iteration:

• https://discourse.gohugo.io/t/39014
• https://discourse.gohugo.io/t/38614

Note that the same differences are present when comparing relLangURL and absLangURL.