When I delete one element on PI System Explorer and I choose the second option for deletion mode ("Only delete this reference to the object. Check in is required to complete the action"), and then I perform an "Undo Checkout", PI System Explorer doesn't undo my deletion.
I'm using PI System Explorer version 2018 SP2
Here are some screenshots:
2) Delete option
3) Before Check In deletion
4) Undo Check out for staged changes
5) After Undo Check out