Thank you, Torres, for the in depth explanation. The decrease in the ability to impart spin on the ball is an interesrting observation. String movement or tension loss would be an easy explanation. Since you are not experiencing either, my best guess (and this is just a guess) is a change in the integrety of the string surface itself from repeated contact. Are you a heavy top-spin player? It is unlikely that you would be able to see any change in the appearance of the string (I'm sure you would have reported it if you did). As you know, Monogut ZX is a very smooth string. Also, Zyex has a higher friction coefficient than poly, which makes it more susceptable to nicks and scrapes - one of the reasons for the detailed strining instructions we provide. I am wondering if this susceptability to friction is affecting the surface as you play and changing your ability to control spin. Of course, it could be something completely different.
Do you have access to a microscope or magnifing glass? If you do, I would recommend looking at the string from your hitting zone magnified and compare to the same magnification of unused Monogut ZX or some from the outer areas of the racket where your don't make much contact. Just an idea.