I saw a user having this issue, and figured out the issue was the app was added as a widget and not put in the icons-only task manager. I think drag and dropping an app anywhere on the panel should add it to the Icons-Only Task Manager to avoid such confusion. Adding QuickLaunch through edit mode gives largely the same behavior as drag-n-drop adding a widget currently does (QuickLaunch does let you remove it from right click menu though, which is much more intuitive).
This is a difference of dragging to different parts of the same panel, and there isn’t really an indication to the user when they’re adding an app as a widget vs regularly pinning to the taskbar.