Oliver Cowdery initially wrote heart in 𝓟; then he corrected the singular to hearts but without any change in level of ink flow (which suggests a virtually immediate correction). Elsewhere in the Book of Mormon there are 13 occurrences of “the hearts of the children of men” but (as one might suspect) none of “the heart of the children of men”.
Summary: Oliver Cowdery’s immediate correction of heart to hearts in 𝓟 undoubtedly made 𝓟 agree with the reading in 𝓞 (no longer extant here).