Siphoning Strike is a strike skill that doesn't generate combo, and it should.

This seems like a bug. The definition for combo specifically says "Combo is built by successfully Striking enemies."

Siphoning strike is by no means an ability I would use in order to generate good amounts of combo, but the fact that it doesn't at all means that sometimes the combo I have can fade because I haven't generated any in the time I have spent using it...

This is just another of a series of things that seem like bugs revolving around combo.