Olbart be like:

if you mean in-universe wise, that's the effect of Gluttony, everyone from that world in particular loses memories of someone who's been Gluttonied, (what I call it), their relationships and memories of the person are just wiped out and not replaced with anything in particular, just rewritten patches of logic. Emilia, Ram, Roswaal, Petra, the kids in the village, Crusch, Felix and Wilhelm all forgot, too, hence why none of them mentioned Rem at all. Emilia naturally lost her memories of Rem because of Gluttony eating her existence. If you mean out of context, then yeah, people don't notice that, but it's more like Emilia just casually thinks Subaru is speaking about someone he met that she's never met before, in the same way your parents might be like, 'who is your friend you met before? that's their name. Ohhh..." Like curious but not fully interested, Emilia's brain instead believes this "Rem" is someone Subaru knows that she's never met, some stranger, because all memories she had of her are gone/purged. So instead, Subaru is talking nonsense about some person she never met before and it confuses Emilia. So she just asks for clarification on the matter, (from her pov, anyway), as you'll see in Season 3 and onwards, when other Gluttony victims do appear, people do ALWAYS forget them except for Subaru, more like the world being rewritten or something.

Yes I knew he was a guy