Perhaps but if adding it to the list costs them little then to do a print run I don't get why they wouldn't and just have two printings concurrently, if they did say Consider Phlebas with the first edition cover I'd pick up a second copy just for that cover.
Though I'm carefully not going to look on AbeBooks to see what the first editions currently go for since I don't need to spend that kind of money (want to yes, need to no).
What annoys me with SF Masterworks is that they changed the style (and checking now, it seems they might have changed it again from last time I bought any). I don't see any point in continue buying the series as opposed to individual versions if they're not going to stick to a cohesive style anyway. I might just finish buying the original numbered style (up to number 74 or something)
But anyway, they've "only" published about 200 novels, mostly older, so it's not that weird that Banks hasn't been added yet. It makes sense if they focus on works that have not been republished as recently until they get through the many obvious candidates.
I think there are 2 different series. The "SF Masterworks" seeme to be fans-liked-it, while the "Fantasy Masterworks" is editor favorites.
You are right that those are two different series, but they did change the design of SF Masterworks around number 74-75, and dropped the numbering as well.
I have books from both before and after. The original SF Masterworks series has black covers with a number clearly printed, while the new one has yellow covers.
According to Wikipedia, the yellow version is from a "relaunch" in 2010 onwards, with some of the original numbered books being republished with the new cover as well. Why they felt the need to do that, who knows.
To make it even more confusing, Gollancz appear to have marketed other series with the same name.
Here's a page that shows a bunch of them in both iterations, though without showing the spine:
https://bearalley.blogspot.com/2019/06/sf-masterworks-cover-...
Here's one showing yet another version (the other version of the "yellow" series has yellow spines as well):
https://blackwells.co.uk/bookshop/product/SF-Masterworks-Mas...
I quite like that one. What I don't like is starting to buy a series and then not being able to keep buying the same design... If it's going to keep changing, I'd rather just have them be individual.
Here's someone on Reddit that has two of the SF Masterworks variations (top left, and top right), and Fantasy Masterowkrs (most of the second row from the top)
https://www.reddit.com/r/sciencefiction/comments/q2eiwk/my_h...
From what I can tell they've done yet another variant at some point, but that's titled "The Best of the SF Masterworks" so I guess that can be forgiven for using a different design:
https://www.redlionbooks.co.uk/product/the-word-for-world-is...
(yes, I care more about this than I should; it's made me pretty much stop buying paper books and instead put other things on my shelves)