I can confirm this bug while using Ride the Lightning, Mist Form or Tornado.
My hp with 30 into water: 17,615
Ride the Lightning: 14,615 (lost all 300 vitality from water)
Mist Form: 13,335 (lost an extra 128 vitality on top of 300 from water)
Tornado: 13,335 (same as above)
My precision with 10 into air: 1016/9%
Ride the Lightning: 916/4%
Mist Form: 916/4%
My power with 0 points into fire: 1761
Ride the Lightning: 1761 (no change)
Mist Form: 1581 (lost 180 power)
Tornado: 1581 (lost 180 power)
My toughness with 0 into earth: 1612
Ride the Lightning: 1612 (no change)
Mist Form: 1484 (lost 128 toughness)
Tornado: 1484 (lost 128 toughness)
I have 2 power/toughness/vitality daggers,
Totalling 180 power, 128 toughness and 128 vitality.
Since I lost exactly that on top of stats from traits when using Mist Form and Tornado, I’m pretty sure the weapon stats got removed when transformed.
I think someone should check using weapons with other stats.
Ride the lightning: Removes vitality and precision from traits.
Mist Form: Removes vitality and precision from traits, removes all stats from weapons
Tornado: Removes vitality and precision from traits, removes all stats from weapons (I know it gives you buffs, but it also removes the stats from traits/weapons. I can confirm this because I was watching the hero panel)