I got this info from another reader!! ( because I haven’t read the manga I just got spoiled :(( )
Well Yashiro wished for Hanako to get out of the fictional world that Mei made and live in the real world but soon Hanako got mistunderstood and thought yashiro meant that she could live longer. so he was disapearing so he can give his life to Yashiro. not clearly he gave his life to Yashiro but yea. also he was fading to a diffrent world. so basically hanako cant see yashiro anymore cause he is gone .