On the part of Russia, anything to publicly indicate or confirm betrayal by India would have political implications that harm the broader arms trade relationship. Given China's growing lead in arms, Russia has no alternative growth markets of serious scale aside from India. I would also question whether the sub involved is a stripped down "export" version of the original Russian design. Just as the Americans neuter and leash most of their exported high-end arms, (e.g. significantly inferior armor and munitions packages for tanks, software controls in fighter jets giving veto power to the Americans, etc.), I imagine Russia would have done the same.