Thick white smoke in a headlight is because a bit of air started getting into the bulb and the filament is burning that gas making tungsten oxide.
Bulbs can leak slowly at the pins on the back side
A bulb can be new and leak or it can be 70 yrs old and still holding tight
I never pay for old bulbs at the junk yard now because too many tested good but had filled with air and smoked and burned out quickly or sometimes instantly on me.
You may as well replace any bulb that has signs of internal fogging or smoking before they fail permanently on the road.
-s