Localized `strings` resources are not detected correctly

I seem to be hitting the exact same issue as #80, namely that my Localizable.strings(dict) files (and InfoPlist.strings files) are not detected correctly and seen as loose files:

As you can see, strings files for storyboards are seen correctly, whereas normal strings files are not seen as language variations of the same file.

The directory structure is correct though:

Application
  - Resources
    - Base.lproj
      - Authentication.storyboard
      - ... (only .storyboard files)
    - en.lproj
      - Authentication.strings
      - ... (other .strings files for storyboards)
      - InfoPlist.strings
      - Localizable.strings
      - Localizable.stringsdict
    - fr.lproj
      - Authentication.strings
      - ... (other .strings files for storyboards)
      - InfoPlist.strings
      - Localizable.strings
      - Localizable.stringsdict

My project file (mostly, the relevant bits):

name: MultiSelector
targets:
  MultiSelector:
    type: application
    platform: iOS
    sources:
      - Application
    settings:
      base:
        INFOPLIST_FILE: Application/Resources/Info.plist
        PRODUCT_NAME: $(TARGET_NAME)
        SWIFT_VERSION: 5.0

Also tried using XcodeGen in another project, same issue, which is worse there 'cause that project has +-10 languages.

Start with the sources entry in the shown project YAML and compare how storyboard .strings files are detected with InfoPlist.strings, Localizable.strings, and Localizable.stringsdict under the en.lproj and fr.lproj directories. Done means these files are grouped as language variations rather than reported as loose files, including when the project is generated from XcodeGen.