Given a .gitignore file:

*
!./PKGBUILD

rg will skip the PKGBUILD file:

DEBUG|ignore::walk|ignore/src/walk.rs:1639: ignoring ./PKGBUILD: Ignore(IgnoreMatch(Gitignore(Glob { from: Some("./.gitignore"), original: "", actual: "**/", is_whitelist: false, is_only_dir: false })))

This appears to be due to the leading dot there.
Git handles this as expected.

Removing the dot (using !/PKGBUILD) whitelists it then:

DEBUG|ignore::walk|ignore/src/walk.rs:1642: whitelisting ./PKGBUILD: Whitelist(IgnoreMatch(Gitignore(Glob { from: Some("./.gitignore"), original: "!/PKGBUILD", actual: "PKGBUILD", is_whitelist: true, is_only_dir: false })))

What version of ripgrep are you using?

ripgrep 11.0.2