This very well may be intended, but here's the problem I see. The siphoning ult starts out as an pbaoe damage and stun. It can be morphed to add a single target damage effect (soul tether), all other effects remain intact as is stated in the morph preface.
However, the other available morph (soul siphon) which adds a 3-ally heal, loses the stun accompanied by the pbaoe damage. This loss of an effect is not stated in the morph preface tooltip.
If it is intended, it REALLY should be put in that tooltip to properly inform players of the ability choice they are making, in this case the loss of a quite powerful pbaoe stun.
Bump for support, or to explain to me why it's ok for this to be happening, contrary to my belief.