@clear_aria I'm not sure how much bigger in scale you would need, going from "something affordable by a not particularly rich medial duchy" to "consuming the entirety of the Sovereignty's budget" to count as a major or even complete blindside.
Sylvester told her to raise everyone's grades and ranks. He did not tell her to insult a prince to his face (twice!) or inject Ehrenfest into the courtship of the Klassenberg princess (and therefore the royal succession!). In year two, Myne was explicitly instructed to limit how much "advice" she dispensed to others, especially royals. She immediately started offering advice to Hildebrand because she felt sorry for him feeling left out. In year three, Myne was again told to stop proactively offering help and limit the scope of her involvement only to the letter of prior agreements. She then went and suggested the royals participate in the dedication ceremony because she wanted to "help" them with their mana shortage.
Sylvester did tell Myne to do her thing with the negotiations with Sigiswald, and almost certainly did so knowing she would likely again exceed whatever remit or constraints he might have tried to impose. That doesn't mean he's hypocritical or otherwise for complaining about it afterward. Intentionally choosing a path that you know will cause you difficulties or headaches, if only because it's the only path, doesn't mean you can't feel annoyance or frustration afterward, since the headaches and difficulties certainly don't magically go away because this was the only option. As I said earlier, Sylvester is venting here as a stress reaction. Hypocrisy only enters the picture if he actually tried to undermine the results of Myne's actions, which Sylvester most certainly did not try to do here.
"Price" entails much more than just coin or resources. Political capital has a price all its own, and Sylvester is needing to expend quite a bit of it in order to make Myne's requests stand, especially in the face of the misunderstandings some of her actions caused. After all, Sylvester could just accept the Sovereignty's demands wholesale and not even try to negotiate terms. Myne's "requests" made prior to Sigiswald however carried a greater degree of legitimacy because Myne had the implicit authorization of speaking on Ehrenfest's behalf, as far as naming Ehrenfest's terms, and her own personal requests had further weight because that same authorization implied Ehrenfest would also be backing those personal requests, and that implicit death threat. Against the royal family.