I installed todoist-thibaut via snap on the command line.
$ snap install todoist-thibaut
The app appears fine in the Budgie menu:
But when you run the app the icon for the app in the Budge panel shows a generic application icon:
Furthermore, this icon cannot be pinned:
I have used this snap with no issues on Ubuntu Gnome, Ubuntu Budgie, Fedora Gnome, Fedora KDE, and others.
It is a very basic electron frontend for the Todoist app created by an Ubuntu employee.
If a change needs to be made in the snap itself, I have access, but I am not sure it's the snap, I think it might how Budgie is handling the snap.