Thanks again for taking your time, I am quite new to this and doesn't know anything more than in the ATPL-book. So I apologise if I don't get the big picture.
I found this picture as a reference,
Even though it's a generic picture, it seems that for pure turbojet (by pass 0) the thrust output dips initially to recover later on at higher Mach speed. My own guess, which I'd like to have confirmed, is that the increased ram pressure is dominating both the momentum drag (loss of thrust) and the reduction in acceleration through the engine (Voutlet - Vinlet).
For a turbofan, the increased front area will produce a great deal of momentum drag why it suffers quite some thrust loss with forward speed.
It's interesting that the JAA learning objectives specifically states that thrust from jet engines should be considered constant with increasing speed.