Card discounts not stacking

[tiford]

I am trying to play Earth Catapult (pay 2M less for cards) and Space Station (pay 2M less for space cards), but their effects are not stacking.

Should they stack? It seems logical that with both of these cards, I should get a 4M discount on space cards but it's not happening

[SpajderDzerusalem]

They do stack. Like this:
https://i.imgur.com/EVzn2na.png

You are probably doing something wrong, screenshot helps.