I suppose the show is trying to say that the taste of Sasakura’s cocktail deteriorates quickly so the while the first sip’s impact was higher for Miwa, if she finishes the glass the overall experience will be worse when compared to drinking Kuzuhara’s glass.

It is hard to think that Sasakura who exhibits such control over his cocktail would make that kind of mistake, other than for plot reasons though.

Kuzuhara gave Sasakura some harsh words, but we got a scene of Sasakura acting harsh towards Kawakami just before, so in both cases they might just be trying to cheer the other on to better themselves in some Japanese round about ways, I guess.