Why is voltage across LED filament bulb slowly decreasing?
Looks like that bulb has an internal circuit that provides constant 3.5W output to the LED filaments, so that it will draw I = 3.5/V where V is the applied voltage (for some voltage in the 12-24V range). 

If you apply less than 12V it may no longer be able to provide full power, but it will likely do so for voltages somewhat below 12V. 

When you add a resistor (and assuming the bulb can still provide full power)  you get I = 3.5/(12V-I*R) which, if you solve for I, is a quadratic equation, and for R > 144/14 ~=10.3 ohms there is no real solution. 

So the bulb will probably hover on the edge of not working and the results may not be consistent. 

If you want it to operate from 12V, you just apply 12V. Any added resistance just wastes power. The bulb is not intended to be dimmed, as per the Amazon listing.