Siphoning Strike not building Combo

Siphoning strike isn't building combo for Tempest Bell or Mantra of Destruction.

The other monk dash strikes (Killing Palm, Staggering Palm, Hand of Chayula) build combo, and the skill gem lets you slot in the Practiced Combo support for chance to build additional combo, with the tooltip for the dash component showing that chance.

Can only assume this is a bug which is unfortunate because I want to use it for building power charges and combo points for a Mantra boosted 7 charge Falling Thunder. Plz help