I haven't tried the Snap (I'm not a fan of the proprietary and centralized nature of the Snap back-end, so it's not a service that I use), but the application list is certainly not shown for the Flatpak by default. The README for the Flatpak specifies that one has to manually use /bin/xdg-open as the path to show the application list.

I installed the Joplin Flatpak from scratch on a system that never had Joplin installed, so this is not the result of a previous configuration.

Perhaps the Snap build has this configuration by default, which makes the experience nicer, especially for non-technical users. I'm asking for all other Linux builds (including Flatpak) to have this better default as well.