According to this, it says that the game randomly uses a decimal form of 20% (0.2) and rounds it down, causing it to look like it says 0%. Don't worry though, it actually is still reducing the time by 20%.
thanks, it's just for me or everyone's seeing that every post says "5 Days ago"? (perhaps it's my bugged laptop)
I don't see anything that says that won't happen, so yeah, you should get a total of 40% faster shout regeneration.