Seems there’s either a display glitch, or the Sentinel’s Helping Hand does not benefit from Healing Effectiveness.
My Sentinel has 114% Healing Effectiveness but the spell (alt) shows 0% (5% per point) gain. I checked my Primalist’s Eterra’s Blessing and it accurately reflects Healing Effectiveness. The tags are the same (HH also has the Area tag for what it’s worth).
On a side note, the Damage Per Second for HH is of course blank, but I would expect a Healing Spell would show the amount of Healing it’s doing.