Apps unable to add Login Items to User Account in High Sierra

I recently upgraded to High Sierra and discovered that some of my Login Items (Users & Groups system preference) have disappeared. Launching the apps and checking (after first un-checking) `Start X when you log in` or `Launch X at login`, etc., these apps still do not appear in the Login Items list. I can only get them to appear by manually adding them.

Anyone else have this issue? Any ideas?

Thanks, Jim