Instantaneous Magic
Demon Booms work by temporarily changing the rules for reality. For example a Demon alters reality around someone so that injuries heal much faster. The target's mangled arm quickly heals into a working arm. The Demon then leaves and reality reverts to normal.
Does the arm go back to the mangled form? Nope, why would it? Now it is just a normal healthy (nonmagical) arm, and the usual rules of reality allow those.
The demon changes reality so that centipedes are spontaneously generated. Loads of bugs appear. The Demon then leaves. Do the bugs leave? Nope, they are just plain ol' nonmagical bugs. The only magic was the magic that brought them here.
The demon changes reality so that a piano appears above my head. I go splat. The Demon then leaves. Do I get better? Nope. Now I am just a splatted person.
The demon changes reality to increase the size of the mana resevoir in Gerard's head. Gerard can now hold twice as much mana. The demon then leaves. Does Gerard go back to normal? Nope, this is similar to the arm example. The mana resevoir is not itself magical.
All demon boons work like this. They create changes. They do not create continuous effects.