Ah, I think I see what you're saying here. When we see Caesar unconscious next to the heart, the heart isn't making the ba-dum/dokun sound effect, so it gives the impression that it's not beating. I will say though that while there are panels where disembodied hearts are shown beating, there are also several that don't include the beating. One page from chapter 724 features a heart in almost every panel, but only shows the heartbeat sound effect once.
Because hearts are regularly depicted without the heartbeat sound effect, it isn't an issue to show this one without the sound effect. In fact, it would be more of a problem to depict it with the sound effect, as that would be definitive proof that the heart was still beating and Monet was still alive when Oda clearly wants us to think that she's dead.
Now, one could make the argument that the way Monet's heart was beating
during the stabbing is indicative of her dying. Because the heartbeat is written as "ba-dum - ba-dum.. - ba-dum..." with increasingly long ellipses and not just straight "ba-dum - ba-dum - ba-dum" it gives the appearance of the heartbeat slowing down.
I personally think that it's more reflective of the scene changing, giving the impression that the sound is fading out as if we were seeing it happen in a film since that would be in line with the broken-up panels that Oda uses to indicate a transition, but it's an interesting detail nonetheless.