After reviewing the code for opening a Contact Sheet of a Collection, it does appear that it opens in Arrangement sort mode intentionally. As for why it was designed that way, my best guess is that it made sense for an arbitrary set of items to default to a sort method that allows for an arbitrary order, even when the default sorting method for any other kind of Contact Sheet is set to something else. That said, maybe we can set up a preference to allow for Collections to follow the preference that Max referred to.
And one final note: in my testing, when I opened a collection where I had not previously manually arranged the images, they were sorted by filename, not capture time. Probably won't make a difference in most cases, but figured that was worth pointing out.
