I don't know if there is any kind of official policy on that. However, with logos you can make sure Roman numerals are clear and look like Roman numerals. In text media, you don't have any control over the font things will be read in, so it is safer to use the number 2.
Well, it's usually a stylistic choice in other games. For instance, The Witcher and Mass Effect series have consistently used Arabic numerals for their sequels, while The Elder Scrolls and Dragon Age always went for Roman numerals. Divinity is much more indecisive about this, it seems. I've just noticed that the original releases of Divinity 2 used the Arabic numeral, but appears to have switched to the Roman one after getting bundled into the Dragon Knight Saga...