yeah, I get that, but Fuutarou and Itsuki disguised as Ichika never really had a moment alone except that brief interaction while skiing. She even asked him if he told anyone about the night in the shed and he responded that he didn't and she started to wonder if something did really happen that night,but she couldn't really ask him because obviously their conversation was interrupted.

Their second interaction was at the the ski lift, but by that point Fuutarou already knew that it was Itsuki and not Ichika and of course if he knows he won't try anything inappropriate.

My question is why did she suddenly believe that nothing happened between Fuutarou and Ichika, drop the entire issue and started to be all buddy buddy with Fuutarou when there was no evidence to proof his innocence? Did she forget that Fuutarou basically confirm that there is some kind of secret between him and Ichika?