LD_LIBRARY_PATH not set for application unless in shell

I’m working on packaging pympress https://github.com/NixOS/nixpkgs/pull/62137, which is an awesome presentation tool for pdfs (presenter& audience view, video support…). However, for some reason pympress cannot find the relevant VLC lib unless I’m in a shell:

> pympress has_video.pdf
[ video fails ]
> nix-shell -p pympress
$ pympress has_video.pdf
[ video succeeds ]

I’m pretty baffled by this, any tips?