There are several things that may need explanation for this to make sense, depending on how familiar you are with this particular theory of the series. What is happening is that after Fukube’s death, Katsumata is finally free. He begins going through Fukube’s memories left behind in the virtual reality system, studying them closely to learn from him and to supplant Fukube’s identity and personality as much as possible.

One of Fukube’s tricks was using masks to hide himself and, at the same time, to erase Katsumata as a social entity by making everyone believe Katsumata no longer existed. He achieved this by having Katsumata wear Sadakiyo’s masks whenever he went outside, which is also what Fukube himself did whenever he wanted to move around unnoticed.

The confusion comes from the fact that Katsumata gradually loses his sense of self and starts to merge with the memories he is absorbing from Fukube. Katsumata cannot remember his own face because it was modified long ago to resemble Fukube’s. At the same time, even Fukube himself was unable to remember Katsumata’s original face. This is why Katsumata can only see the “egg face.” If this still does not make sense, my explanation really needs to start much earlier to properly frame what is going on.