Anyway, for some reason "Go to Related File" with a custom relatedFilePath worked in beta 10, but not thereafter. 

I've setup a test project, uploaded here: https://github.com/textmate/bugs/files/349542/textmate-related-file.zip

When you open the project with beta 10, and open the `test.rb` file, `Go to Related File` will take you to `test_spec.rb` and from there `Go to Related File` will take you back to `test.rb`.
In any version after that, eg the latest beta, 11.4, this won't work.

Has something changed with the format of `relatedFilePath`?

Thanks,

Koen