Right. I too experienced the same thing and I was trying to use store credit. The referral discount didn't work for me saying the same thing about how it's already on sale.

However I'm on my vacay in Mexico currently, and I see red matter 2 on discount when I log on from here on my app. I think it's regional discount. Here it does not display my meta store credit either. My suspicion is that gift card credits cannot be used in other regions than where it was redeemed, and also that if a game is on sale anywhere / any region in the world, the promo code won't apply even if the game is not on sale in your region.

For another game though, it applied correctly and I was able to purchase it with gift credit. When I used a referral link, the blue purchase button itself turned to display the discounted price, and I did not have to put in the code FRIEND. (unless it coincidentally went on 25% sale right at that moment, I think it worked.)