I tested this by using disintegrate and 20 mana regen to equalize my mana loss/gain so the number wouldn’t move, and then used mana strike and entered disintegrate while the mana ORB was traveling to me.
Apologies if the text is a bit small, my uploader shrunk the footage for some reason. Still, one can see that each uptick of mana is only 2, not 3, which would be expected if the value was actually 70% less given I would get 10 mana by default. This is actually quite significant as that’s 33% less mana gained than expected.