Roxas would have been worried by Saix's remark, but together with that gesture, it hits him differently. Motioning to his heart like that, like there's something there that he's not used to? Maybe Roxas is jumping to conclusions. Axel seems to have missed it.

"It- skipped me this time," he confirms distractedly, looking back and forth between the two of them. "Does it- seem like you're feeling things more than usual? Or- not as easy..."

As the case may be. He remembers what it was like to have Vanitas's incomplete heart, and what it was like when a curse had given him a heart of his own. He touches his own chest, reflexively.