Project information:
Name: Trilium
Homepage: https://triliumnotes.org/
License: AGPL-3.0 license
Repo : GitHub - TriliumNext/Trilium: Build your personal knowledge base with Trilium Notes
Old version: https://flathub.org/en-GB/apps/com.github.zadam.trilium
Old Repo : https://github_com/zadam/trilium (Redirect to New)
Upstream has been contacted: yes, they failed to do it, most probably lack of skill.
The team is doing really great work updating the project, and they have improved it significantly. They also have a very good looking website and docs, but they lack the skills needed for Flatpak and Flathub. they have working .flatpack file build, it took multiple trail and error to make it work with shortcut and icon.
Related issues 
https://github_com/TriliumNext/Trilium/issues/5108
https://github_com/flathub/flathub/issues/6715
https://github_com/TriliumNext/Trilium/issues/4732
Note: new user can only add 4 links so added _