Darth Vader had time with Leia in the detention cell and then in the control room before blowing up Alderaan

He might have felt something, but didn't know what it was since he didn't know that his child had survived. Or he was in denial that he was feeling something. Or his anger, and hatred of the rebellion clouded his senses.

He recognized when he felt Obi-Wan because he had been around him many times before.

It's the same reason he didn't know Luke was his son until The Empire Strikes Back which is about three years later. It's because he thought his children had died with Padmé.

because Lucas had not ret-conned that part yet

Because Jar Jar hadn't used his Gungan mojo yet.