The problem doesn't lie within the "Send Event" action (so you can ignore that one), but with how the "Mouse Pick Event" functions: It only checks where your mouse is currently at and sets that in relation to the specified GameObject.
If the raycast that gets shot from your current mouse position hits a collider/trigger on that GameObject, it fires the 'Mouse Over' Event, in any other case it fires 'Mouse Off' (the 'Mouse Down' and 'Mouse Up' events can also only fire when the mouse is currently over that GO).
It fires 'Mouse Off' whenever it can't detect the GameObject, which can be caused by several things:
- the 'Ray Distance' is too small / the GameObject too far away (unlikely)
- the targeted GameObject doesn't contain a collider or rigidbody
- you need to specify the layer of the targeted GameObject in the 'Layer Mask'
So you should really only use the 'Mouse Off' event option when you are certain that the mouse is currently over the desired GameObject and want to detect if it moved away from it.