It's well known that Muichiro is 'cold' and 'emotionless'. Someone who never feels anything other than indifference.

But even with Genya's scolding, Kanao's talk, and Tanjiro's sighs, no one in the group could change your mind.

Muichiro of course knew, as soon as Inosuke found out he ran off and told him, but instead of confronting it he just ignored it. Acting as if you didn't feel the way you did, treating you as he treated all his other friends, cold.