
I'm also not sure how this actually happened maybe they'll reveal in future chapters? But most likely maybe Osiris tampered with the mirror intending for Seth to see him when he picks it up but nephyths ended up using it and in case of hathor I'm not sure why horus didn't see her as she gifted it to him and saw Seth luckily

I understood that Isis saw Osiris because he was like the previous love (so the part that loved Seth was stuck in the mirror, and the one loving Osiris is now out). For Horus, only one love in his life (Seth, path and present) so he didn’t switch and remain in the mirror (because he saw the same person).

In chapter 17 Hator was looking in the mirror but Seth took it from her and looked into it and in the same moment Horus was looking into it too, so Horus saw the one he already loved instead of Hator. Osiris must have had been looking into the mirror when Nephthys saw him. (really hope this makes sense lol)
My guess is he wanted Seth to look into it and "fall in love" with him..

You could be right, because I think the only reason why Osiris gifted his gifts to his siblings was so it didn’t look like he was interested in Seth, if he just gifted Seth all the presents everyone would suspect him so he had to act like he was sharing his gifts with his siblings and Osiris probably knew what the mirror could do and that’s why the mirror was found in Seth’s gifts so Osiris thought that Seth would go through all the gifts and have him look in the mirror so he can fall in love with him but luckily, Seth didn’t

Now I remember!! It must surely be this. When hator gifted her mirror she was sneakily trying to make horus fall for her so she was gazing at it on the other side but Seth took it n horus saw him and bonus is hw alrdy loved Seth but in case for Seth n Osiris Osiris must've been trying to do similar thing but nephyths saw him insyd of Seth
so the memory of nephtys who loved seth is stuck in the mirror, and the nephtys outside is nepthys who only remembered she loved the one who she saw in the mirror, which is osiris. and that's also why the scales of maat didn't tip. it's like the mirror brainwashed her omg.