It might make sense if you look back to the past chapter. Seiran always put a strong front to everyone except Ari. She only exposes her traumatic experience of killing someone to Ari, not Mimi or any other people. Maybe Seiran's will power is too strong for her own good. It is understandable that she did not reveal her wound to Mimi.
As Mimi has said earlier, Seiran is the total opposite of Shiina, so I don't think all the characters except Mimi will be killed off at the end of the series. (I am not jinxing it)
Now Mimi understand the loss of losing a friend. I am curious to see what will happen next chapter. I hope we will get more Shiina, we need more motherly love in this series.