I have noticed the kids at our gymnastics club tend to stand and walk with good posture, and will even generally sit with good posture when chatting to people

This is part of the training. My wife (ballroom dancer) is the same.

but that all goes out the window if they're hanging around by themselves or sharing screens, then they can slouch with the best of them

And then she does this when she is with me.... it annoys me :P