I assume you have read this on a forum such as the one here, as such Neff has not made a public statement about this claim.
As such I cant comment if this is indeed the case, but if you feel the unit is not operating as per the spec's in the manual I suggest this is treat as a fault.
If I were to hazard a guess at why the power reduced after a period of time, my guess would be due to a safety feature detecting a build up of heat or current overload, and dropping the power to overcome this.
If you would like to proceed with a service claim, can you confirm the product is under 12 months old?
I cant not tell you why it would be done, this would be something only the design engineers in Neff could tell you. As I cant answer your question I will opt out.