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.