MATLAB considers graphics objects to be layered. The top-most object that is eligible to receive mouse clicks is typically the one that receives the clicks, unless objects have been specifically configured otherwise.
imshow() creates image() objects, and image() objects are eligible for clicking on, so they typically "hide" the underlying axes.
Lack of a configured mouse click callback for an object does not mean that the mouse click is passed to the next object down for eligibility: instead it means that the object is considered to have accepted the mouse clicks and discarded them.
The behaviour can be controlled by HitTest and PickableParts properties; see the link above.
