Since the V channel is changed for original image with a gaussian mask the output has been changed. This is because while converting to RGB from HSV, each of the R,G,B channel are inter-dependent on H,S,V channel and even if only the v channel is changed in HSV it will affect the entire RGB image while reconstruct from HSV. In conclusion the output will be different even if only single channel out of HSV is changed and hsv2rgb is used to convert back to RGB format.
So, this is not the consequence of taking the image to HSV domain, because if you don’t change the V channel and reconstruct you will get the output same as input. It is just that some processing is applied on the image which will change the appearance.
Hope it clarifies!