Isekai ni Teni Shitara Yama no Naka datta. Handou de Tsuyosa yori mo Kaitekisa o Erabimashita - Ch. 30

[jenev]

But it was mangaka that asked us, in that last panel. Not mc.

Don't remember which chapter, but it was the one where we got to watch the heroes for a bit.
Like that other user said though, it is not clear that the replica is fake. For all we are told, mc could be the fake one. Or neither. Or both.

But mangaka is the one that drew that prior chapter. So they should still have remembered what they drew, imo.

It's simply a What's Gonna Happen Next?! that editors and publishers put at the end of a chapter to hype the next release and remind readers of things. It's no different than an anime having a next episode preview at the end of an episode.

And it's an extremely common practice.