UV Editor: unwrapped faces disappear if I select other faces I'm trying to draw a 3D mushroom following this guide: https://www.raywenderlich.com/49955/blender-tutorial-for-beginners-how-to-make-a-mushroom

I need to paint the head of the mushroom and the stalk separately. For this reason I need to unwrap the stalk and the head of the mushroom and to group them on the UV editor. This is the the result that I should get:



Like you see on the UV editor there is both the head and the stalk of the mushroom. But if I try it myself, once I unwrap the head and I get this result:



If in the 3D editor I deselect all the faces (with the goal of starting to select the stalk's faces), the unwrapped mushroom's head on the UV editor disappears. Why? isn't there a way to "save" the faces unwrapped in the UV editor? how can I unwrap two set of faces without that one disappears?
There are two things going on here:


When you hit U > Unwrap, Blender only unwraps the selected geometry.  To unwrap the head and tail of the mushroom, select them both before unwrapping.
The UV/Image Editor view only shows the pelt[s] for geometry that is selected in the 3D View.  When you deselect something in the 3D view, it's unwrapped texture coordinates aren't deleted, they are just no longer visible in the UV/Image Editor.You can optionally use Sync Selection option so the UV selection matches the 3D view-port. This can be toggled using the button to the right hand side of pivot point popup.