Spellthief's Edge (bug) - Brand's Passive vs Gangplank's Passive

Brand's passive triggers Spellthief's Tribute. Gangplank's passive does not.

GP's passive triggers tower agro if he walks into tower range while an enemy is afflicted by his passive.

Why does Brand's passive apply Tribute, but GP's does not?

If it is because GP's passive is proc'd off of an on-hit effect and Brand's passive is proc'd off of a spell, then spelltheif's should at least state this. Alternatively, just because the way the abilities are proc'd are different, the abilities should be treated similarly for the Tribute passive.