There were several indications in the movie that the new replicas had some sort of issues, including the mother feeling sick while running, son cutting himself, son missing his plate, i.e. hinting at some errors either with the body or the mind rejecting the body.
And it went nowhere in the 3rd act of the movie. Literally dropped that plot point. I was expecting that Checkov's gun to fire but it never did. So weird.
Can someone explain this to me
We can write it of as a lack of muscle memory kind of thing, and so time would solve those glitches. Perhaps it was meant as a way for them to feel their physical infancy, so that they would earlier accept it as the truth, once he told them (or her). Her running pain was the first sign she felt... I mean if she would not feel any different, why on Earth would she (or anyone) believe it. There had to be some anomalies, to make their acceptance even remotely believable.
shareI noticed this too, especially with the son. He appeared to have some sort of malfunction he was starting to experience. I don't think there is anything to explain though. My guess is that this is one of those cases where there was some sloppy editing, you hear about (and see) this in some movies. That there was some sort of subplot about this that was later removed from the film, and didn't make the cut. And these scenes were what remained of it.
Or possibly there were some rewrites, and this didn't get removed. I definitely think there is something missing in the film that would have explained it.
I also think the part where the mother was holding her chest after running was her subconscious remembering being skewered through the heart in her previous life.