Hi @Ritesh_Rijal, I just learned something… and deleted my first post because it was probably useless. Here goes…
In your home directory, navigate to .config/autostart
That’s ~/.config/autostart and you can do this in Caja by showing hidden files and directories.
Are the applets there as desktop files? I’m betting they are. Like blueman.desktop.
It turns out when you delete an entry in Startup Applications a copy is placed here with a “hidden” line added inside.
Believe it or not, deleting blueman.desktop from ~/.config/autostart may unhide it. Restart Startup Applications and it should now appear as its default setting.
It turns out the master copies of these exist in /etc/xdg/startup and they are overridden by the copies in ~/.config/autostart. Strange but true.
Does it work?