promotional art is always official art.
tl;dr - Touhou is not strict regarding fanwork, thus it's easy to create such fanwork and draw promotional_art for it, and I believe fanworks do not fall under official_art, unless we rewrite the vague wikis of promotional_art and official_art to consider fringe cases like Touhou (which given its significance in Danbooru's history cannot somehow exclude it from either of these tags + other uncommon scenarios similar to Touhou's case).
I will use Touhou as an example because it's basically my only interest and ZUN imposes very lenient guidelines towards the creation of fan works despite it being somewhat of an outlier in this context. The official_art wiki does not provide clear boundaries as to whether art from works where the base copyright creator did not have a direct role in the development of said work should count as official_art. In Touhou's case, art drawn by ZUN himself and by artists he hires to draw art for various official works (because he writes the story so it follows canon) is official_art. To quote official_art's wiki so this paragraph makes sense if the wiki ever gets revised to mean something other than what I am understanding: "Artwork that is made by the official company/artist".
My knowledge of Touhou Lost Word is very limited, so I don't know if ZUN himself had a say in its creation or officially endorses it, but I view it simply as a commercialized fangame made by a third-party company and thus I think none of its artworks should count as official_art by the wiki's current description (see the last sentence of the first paragraph; ignore if there's evidence ZUN actually worked with this company meaning they're second-party, meaning TLW is official). Therefore, promotional_art of Touhou Lost Word should not have official_art implicated to every picture. In a similar fashion, because there are myriads of Touhou fangames that follow TSA/ZUN's guidelines, this means every art promoting said fangame will count as promotional_art and this implication suggests that they are official_art. This is absolutely absurd.
I can easily put together a small team to shart out a Touhou fangame with a big chungus artstyle and then draw promotional_art with this artstyle and then upload that art to Danbooru and apparently my fangame's promotional shit with characters drawn like this gets official_art slapped onto the post. Are these the "fringe case" you are talking about?
As I've stated before, Touhou is a unique case in this context as Team Shanghai Alice consists of just humble & generous ZUN as opposed to a large company who will sue your entire bloodline into poverty if you try making a penny off your Azur Lane/etc fangame. Most companies are this protective of their IP so their promotional_art will be created first/second-party and will be official_art. However, I'm sure there are more examples other than Touhou where this is not the case, but I'm open to criticism regarding my thoughts on this matter as I'm thinking with a very narrow mindset + I worry I'm misunderstanding something here.