I create the marriage fact myself about 98% of the time. I'm finding that sometimes, Ancestry uses the fact I create when I find a reference, and sometimes it doesn't. And, sometimes, it changes the title from "Marriage to John Smith" that I created to just "Marriage." Sometimes, it creates a new marriage record with the same date that I entered. In that case, I have to manually go in and adjust the sources and details to the one I created. If later sources come in, it will create a new marriage record instead of using the one that is already there.
I have not done enough research and tracking on it to tell if it is just certain databases that work oddly , or whether it is the matching process itself that gets confused.
Best advice is to check your overview page after adding any sources to make sure they go where you want them to go.
I do have the same problem mentioned about 2nd and subsequent marriages. They can get really messed up at times.