There appears to be a few issues with the discount showing up, there are a few threads / posts about this from others too.
If you have cleared cookies and browser cache, made sure you are logged in etc and it's still now showing up then all you can do is contact support and see what they say, I'm sure they will extend the offer if it's an issue on their end.
Yeah, the Tech-Support from ticket I wrote because of the other issue where I wasn't able to log in (when I started Gamemaker 2)
messaged me, but then it was already fixed. So I told him about the issue regarding this and he said it was a mistake on his part and it should be fixed now.
And now I see the discounted prices.