all shapekeys are edited at once Im very new to the concept of shapekeys, so I dont know, wether this is a bug or a mistake in my hands.

I modelled this heart and tried using shapekeys as a way to animate the cardiac cycle.

I am in the standard position of the heart.
no vertex groups, no settings regarding the Object Data tab.

I hit the plus button to create a basis shapekey

only "Relative" and the box behind the name is ticked.

I hit the plus button another time.

I slide the value to 1.

I sculpt a bit.

Whenever I slide the Value factor, nothing happened, because my basis shapekey seems like also edited.

How can I solve this issue?

download .blend here
Actually both the 2 shapekeys are identical, the object is deformed because you've given it a Multires modifier. So delete this modifier or at least deactivate it and make the mesh deformations you need in Edit mode with the shapekey called Key1 selected.