I tweeted Virgin's account, and the response was a link to the terms and conditions of the complimentary upgrades (on the Platinum Status Benefits page), which in point 4 or 5 state that the upgrades are subject to the availability of Business Reward seats. So it is stated there. Makes the whole point of this rather benefit rather mute going forward, because booking flexi does not guarantee an upgrade at all (complimentary or using points) - you actually have to spend the additional extra time trying to call their membership centre and inquire about the possible flights, or trying to make a dummy reward point booking. Time consuming, and in my view hard to make use of - albeit emotional as I just got off this flight and I had a really coughpy seat with the ancient entertainment unit housing under the seat in front of me and I couldn't stretch my legs.
As I previously said PM VFF here, I have done this with the same issue and had a successful outcome.