How does mana reduction passive pair with mana reduction ability? Narcisa and Jove against Mana buffers?

Does debuff from passive stack with debuff from special skills?
Narcisa’s passive reduce opponent’s mana gain from ss by 50%.
Jove’s debuff reduce opponent’s mana gain from ss by 75%.
Scenario: Fighting the next2b nerfed Myoin-ni tank, if I bring both Narcisa and Jove to war, when opponent’s Myoin activates, does Narcisa reduce the mana gain to 16%? After I fire Jove off and get the debuff on, does Myoin’s mana gain for opponent’s team further reduce to 4% each turn? Or, does it only affect direct mana gain from ss which makes it useless?
Comments and thoughts are highly welcomed and official clarifications are highly appreciated!

Thank you,

Player A