Models are kinda buggy currently. A quick trick to workaround stuff is doing this in your post-update:
Code:
element:SetAlpha(.51) -- this can be anything which isn't what :GetAlpha() currently is. (element:SetAlpha(element:GetAlpha() + .01) would also work.
element:SetAlpha(.5) -- Set the value we want to have.
This forces the C code to update the alpha on the model. The same method can be applied to everything[1].
[1] Everything on the PlayerModel that is, not everything general.