Devs,
Played around with the Healing Aura some more today and discovered what makes the aura decrease in healing when switching weapons.
It appears that swapping weapons makes the game think that empower healing is no longer stacking with the aura, even though the feat is still active. By turning empower healing off, then on again, I was able to get full value heals with the new weapon.
I hope this helps you solve this bug.