The mirror and mirror Nephthys were shown AGES ago... The problem's that we didn't hear about him again for like, a million chapters. So it was more of a problem of not bringing them back up in the story sooner. Also, this new "power" of the mirror, of changing the recipient of someone's feelings, DID come somewhat unnanounced (we knew it could increase the intensity of someone's feelings, so it was to be expected it could also reduce it, but... changing the recipient altogether?? Where the heck did that even come from??) ...Even if readers were right about there being hints about this, in the hieroglyphics, that doesn't replace the necessity to give foreshadowing of such an important plot twist in the story proper (moreover, not everyone is gonna pick up on the hieroglyphics being a "thing")
The art is VERY beautiful and the characters are very compelling but like, honestly, it feels kinda last minute that this MAGIC mirror appeared that Osiris used to trick Nepthys and start all the conflict. Its giving the author wanted to make osiris a villain this whole time and she just didnt get around to actually thinking about how to make it happen so she literally made up "a mirror that shows you your true love", like damn no foreshadowing or anything? Just a random ass enchanted mirror? Okay whatever girl