My understanding was that because the 8th day is d’rabbanan and Gebrokts is a safek and safek d’rabbanan l’hokil.
My understanding is a bit different (although it could very well be wrong).
My understanding is that the 8th day is a safek* and g’brochts is a safek (i.e. we’re not sure that there is uncooked flour…). For a safek, you are machmir. For a double safek (s’fek, s’feika) you are not.
Not that it matters to me all that much — I eat g’brochts all of Pesach.
The Wolf
* Of course, today, we know that the 8th day is not really a safek anymore now that we have a set calendar (further reason to be lenient in this regard)… we only keep it out of minhag.