Can't say that I have heard anyone else claim that DVR is cutting off the end of a non-live show that runs long.
Pretty sure no DVR can extend the recording of a live show that runs long (ie a football game).
On Demand FF is controlled by the content provider. I belive people have stated they see similar on other providers.
And in fact, for a live broadcast the DVR will prompt you by giving you the opporunity to extend the scheduled recording time beyond what the schedule says in case the live event runs long.
Oh yeah, and On Demand can't fast forward... .
You will find out rather quickly that the content providers control this and it is the SAME for all cable/satellite companies, including DirectTV.