A Monster cannot go above 10 Life Points unless a keep Power card says otherwise. If a Power card allows a Monster to exceed 10 Life Points and that card is later lost or discarded, the rulebook does not specify what happens to the excess Life Points. There is no rule in the provided context about reducing Life Points back to 10 if the card is lost.

Source: page 6 — "Power cards do not let Monsters go above 10 , unless a keep card says otherwise."

I don't know based on the rulebook. The rulebook does not address what happens to excess Life Points if the enabling Power card is lost or discarded. To answer, a rule specifying the handling of Life Points in this situation would be needed.

Source: Core Rulebook (pp. 2, 3, 4, 5, 6)