Why is IndiGo adding A320NEOs more aggressively than any other airline?
It's quite common for the first customer(s) to serve as a proving ground.
Memorandums of understanding (MoU's) are signed that detail the specifics, which in 99% of the cases are not revealed to the public.
If IndiGo was upset, it would have been loud, but it hasn't been so:

IndiGo did not comment when approached by FlightGlobal.
In a statement to FlightGlobal, P&amp;W says that it is supporting GoAir and IndiGo “to assess the situation and minimise any disruption”. However, the engine maker adds that the issues “are still under review so it would be premature to speculate on the cause(s)”.
&quot;We are working closely with our customers and our suppliers in order to address and resolve these issues quickly,&quot; it adds.
— FlightGlobal

Not commenting hints at a non-disclosure agreement (NDA) in place and quite possibly discounts.