Do you have the same issue with the AddToCart browser events? If you do, then I would check how web events are set up. Because in this case, the issue might be in the web AddToCart events or dataLayer.
You can also compare the number of AddToCart events in GA4 and FB. If GA4 reports more AddToCart events than FB, most likely FB server tag for the AddToCart event is not triggering in some cases because of an error.