So I discard Lazuli with Magnacarrier and I have Gem-Knight Garnet in the GY but Lazuli did not activate its effect, is it a bug?
The discard effect of Super Quantal Mech Ship Magnacarrier is a cost not a card effect.

Because costs are distinct from effects, if a card is moved to a different location due to a cost, cards and effects that can only be activated when the card is moved by a card effect cannot be activated. 

An activation cost is a mandatory action that must be done to activate a card or effect, separated from the rest of the effect by a semicolon (or the word "to," in most cases without PSCT), which must be paid at the beginning of the effect's activation. The cost itself is not activated, cannot be negated, and occurs at activation time not resolution time.