There are many relevant issues that have been raised in this query by you.
The first is the practice of Group Health Insurance provided by Corporates in India: As you have discovered, there is no uniformity in practices. Depending upon the premium paying capacity of the organisation, the past practices, any international benefits philosophy etc contribute to the type of features that are provided to their employees
The seconed issue relates to continuity and portability of benefits. Most insurers are divided on this. There is no comprehensive guidelines provided by IRDA, the apex regulator, on this issue
Not all corporate medical policies need to have or have the same features. It is a benefit for the employee not to be contrued a right.