Heart Flux passive become useful when used in Infinite Illusion - Heart (Mystery)

Combine LM with Heart Flux passive & Kushina Christmas passive (which also almost the same as Heart Flux), your main damage dealer (usually Madara ETR or Pain Sage of 6th Patch) will get more buff every round whenever mystery is casted (not necessarily executed, meaning if Madara ETR cast mystery but interrupted it is still considered used & he will get the buff)

Other than this, i donno other use for this passive