It appears that you could use a new support under that section of the decking. The heavy foot traffic, and the lack of support under that spot is most likely responsible for the crack. The plank will either have to be replaced, or, if you dont mind the crack, and it is finally supported, you could just tack it down to the new support and leave it.

Here is another pic. It’s a wide space. So basically I would need it to be reinforced underneath? Should the whole board be replaced (in addition to reinforcing it)? Or could we patch it with something?

Deck boards in general on a 45 degree angle require 12" O.C. joist framing across the entire span. Composite decking may need closer than that. If one broke because of the spacing more will break especially is heavy traffic areas. Replacing boards without getting them more support is not enough. Looking at your pic I think I see boards dipping currently, that would be the telltale sign that there is not enough support. You are not effectively going to support all the deck boards without removal and reframing the substructure.

Well….that sucks! So in other words, whoever installed this deck did a shoddy job? That’s the ultimate theme of this house lol. So basically we need a new deck? 🫠

Definitely a conundrum as in high traffic areas this will most likely continue to happen. Up to you how you deal with this and of course I cannot guarantee this to be the case but all I can go off of is the pics and description you share.

Looks like they just did a ”replacement” of SYP with composite, and didn’t bother to read the install requirements for the composite. If they did any other ”improvements” to the house ”for resale”, you might want to give them the side eye too. That is flipper territory.

"So in other words, whoever installed this deck did a shoddy job?"

Apparently.

"So basically we need a new deck?"

No, you can remove the deck boards and relay them perpendicular to the framing which shortens the span and keeps them from failing.