Sorry I closed before Danielle could chime in. My main experience with French Catholic records is with the Seychelles. They were very explicit about whether a child was born out of wedlock and then legitimated by a subsequent marriage. In some cases this might involve six or eight children.
So to paraphrase you, the circumstances of the birth of Antoine jnr were known and, apparently, accepted. So, where does that leave us? An aberration we can't fully explain? An example of something that was not that unusual?
I guess I need to look closer at Rosalie's background. She was supposedly born at St-Eugene, which is at least on the south side of the river, but not particularly close to St-Philippe.