There is also such a thing as redesign of components...again it’s been 10 years.Doesn't answer the question why and what exactly. All I was asking about the nature of the problem and how they tackled it. It seems like we only know that they used to have a spool up issue and judging by the fact that multiple iterations are there, they'd come up with a solution.
The solution itself needs to be known. If they are suffering from a FADEC related issue and if they've solved it, then that mean all is well (kind of).
If it isn't FADEC but the design of the engine itself (compressor stage) then that'd mean they are solving the issue by seeking to tolerate certain operational shortcomings. If the compressor can't handle the sudden increased thrust demand and takes time to generate thrust, it is obviously going to affect the flight performance of the aircraft. A pilot may seek for increased throttle but the FADEC won't translate that request fluently, instead gradually increasing it owing to the limitations with the compressor design. Also, other factors like air density (altitude) etc would come into consideration.