It seems there's been issues with the download of number posts we get from the endpoint used by getPage. Old posts are being missed quick often (with no fix in sight).
Here's the thread if anyone is interested: https://developers.facebook.com/bugs/1838195226492053/
As a "solution" Facebook introduced a new endpoint to get published posts from a page.
Granted it seems to work only if you have a page token, but better than nothing...
I think a getPublishedPosts function would be useful, and it should be relatively easy to add...