Only if we take it according to modern sensibilities. Remember the setting and people are in a place where everyone is generally very conservative, and what more, the aristocracy have certain personal protocols for personal space and distance between people, especially with the opposite sex. The royals even moreso, and the Prince would have received that orientation as part of his education.
Going into his highness' condition, signs suggest it is running a bit out of control, and it rather does bear a resemblance to the Devouring in Ascendance Of A Bookworm. But rather than pain...the impression I get is that it's affecting his mental state by either affecting his hormones, or intoxication and directly affecting his mind. If the latter, then his condition is essentially getting mana-drunk. Like alcohol, his magic has to run its course through his system, but over a longer period of time, and nobody in the palace seems capable of diagnosing that.
Based on context, rather than his fever being dissipated, it just seemed to me he didn't have a fever at all. At least not in the physical sense, and Lia already suspected that just from looking at him.