Afterlife IV Loading uses a React interface with SCSS styling. Its FiveM manifest points to ui/dist/index.html and lists built CSS/JavaScript, PNG images and MP3 music. A source checkout without the expected built files is not enough to display the loading screen.
Setup and checks
Check those paths in the downloaded package and inspect client.lua before enabling it. The manifest enables manual loading-screen shutdown, so test the transition into the game as well as image and audio loading. Keep another loading-screen resource disabled during this comparison.