Eh, fiction is all about lying. The question how good you can write the "no-no, this is THE REAL TRUTH" - stuff^^. And one of the recurring theme of the series is, that you have to discover the lies of your forebears, so I can still forgive them some hickups. People lie in this games, a lot.
No, this is not NPC's lying, it's just the developers being more interested in whatever the latest idea the writers came up with and deciding that is the new canon, never mind whatever happened in other games.
This is not an attack on Larian or the writers. That approach gives a lot of freedom to the writers to do whatever they want, which makes them happy... but it means world-building is uneven.
Or they have finally the budget to flesh out the universe as they want and the hackened state of Rivellon before was the result of lacking said budget. But compared to the Ultima series (who I swear to god, altered their universe every time), the changes were minor and little. The Demon God is a bigger detail, so when they retcon it in the next game, well than I agree to you. But to be honest, this was such a minor thing, I'm impressed you still remember it.
What was a major retcon in my book, was the endfight between the Black Ring and Lucian armys (if we can trust the source), where they told us that Damian died (of course he didn't but if you remember Beyond Divinity a few years ago, the fight between him and Lucian was very different, from what we 're told in D:OSII). That gets way more on my nerves, because you wouldn't need Damian for that. It wouldn't have cost them much effor to write "The Black Ring lost their leader, but still marched on" and that would've be it. But it still can be a devise, to fill the hole for Divinity 2, while still making it interesting for new players (because let's be honest, "I killed the whole army, while they were without the big Antichrist" isn't a very exciting read and makes the Divine look like an idiot).
