she didn’t give birth twice — the “yeon” that died was a body double (a maid who looked like her) and was impregnated by the ml’s cousin around the same time the real yeon got pregnant. the villains then used that to their advantage: took the real, still pregnant yeon and threw her back into the demon lair pit and replaced her with the body double who gave birth and died in the palace all while ml is away. that’s why it looks like she gave birth and died in the palace, but the real yeon was the one who gave birth after that.
though i do agree, the pacing is a little :/
the child the real yeon gave birth to is alive! yeon gave birth to her in the demon pit and while it wasn’t a good environment at all for a newborn, they were to able to make it out (as we see yeon is back on the surface) — while they’re fighting the war, we don’t know where they’ve stashed her away but i imagine we’ll see within the next few chapters
she gave birth, died, gave birth again, came back to life, fought for and won a war in 15 chapters what is this pacing