Theoretically Rozemyne's mana should have started diverging from Ferdinand's, once her mana clumps had been dissolved, since according to what Rozemyne read in the Grutrissheit, it was those clumps that acted like foreign feystones, that kept the dying in place, well beyond the natural duration. There's also a possibility that whatever Anwachs and Erwaermen did to Rozemyne, further changed her mana, to diverge from Ferdinand's.

So between the divine changes and the march of time, I wouldn't rule out the possibility that Rozemyne's mana is diverting more and more away from Ferdinand's as time passes.

Another explanation could be that Ferdinand is in emotional, rather than physical pain in that picture. Giving his name again with the very same stone, is bound to dredge up memories of his father and getting connected to Rozemyne in particular, who we know to be quite emotional when family members are involved, isn't going to improve his ability to control his own emotions either.