Usually, an increase of maximum stat also leads to an increase of current stat. Since Manaflow Band is meant to not follow that rule, it is made to first deduct the amount, and then increase the maximum mana right after. This effect is considered mana expenditure for effects that trigger upon expending mana.
If the deduction in mana leads to a negative amount, Manaflow Band will actually increase current mana, since the floor of the stat is 0. The deduction will effectively be lower than the successive increase of maximum mana which will then increase current mana.
New Effect: Hitting an enemy champion with an ability permanently increases your maximum mana by 25, up to 250 mana (15 second cooldown). After reaching 250 bonus mana, permanently restore 1% of your missing mana every 5 seconds.
Removed Effect: Your next ability with a mana or energy cost has its mana or energy cost refunded, and restores 8% of your missing mana or 8% of your missing energy.