Takumi is changing and he's actually super unnerved by it.
The part that's unclear is exactly how - and when - Takumi's personality had started changing, as well as how much Yugamu had projected would happen. But even though Yugamu didn't expect the drug would alter him to this extent, given his diagnosis it's fair to say that he already expected that Takumi would change in some way- presumably, into someone who would not be held back by doubts and would by extension, fight back against any doubts that arose.
One of the intended effects was to 'raise his confidence', after all, which would have already impacted his cognitive handling.
Nozomi's warning (foreshadowing later), and Takumi's reaction to not only display sunk-cost fallacy but rationalise by invoking Karua, could suggest that the more advanced change was underway. It's only been a few days since this discussion, that Yugamu informed Takumi about the drug's full effects, and he was confronted earlier with the possibility that isolation is going to massively screw with judgement.
Which is not... something that's easy to reconcile, for someone whose MO to 'think with a clear head' is to go back to his room and 'centre himself' before going on a round of distractions to allay his anxiety...
