Thanks for making the temperature measurements. I'm also surprised that the results were so similar.
I guess the heat has no more reason to migrate through the plastic to the surrounding environment than it does to migrate through the porcelain.
Resisting the temptation to split a hair or two.
Please, split. I'd like to learn...
You will be sorry you asked! If we had several very accurate measuring points and sampled them frequently to build a picture of the temperature drop, it is likely that every dripper would have a unique temperature drop curve.
Every material has a fairly unique property specific heat capacity, which is the amount of Joules required to increase the temperature for a kilogram (engineering) or a mole (chemistry) of the material. Multiplied by the transmittable mass (mostly wall thickness, but the transmittable range increases with time and thermal conductivity) gives a nice idea of how much energy it takes to warm something up, which is how much the water temp would drop to get that energy. But we aren't doing this instantaneously, or in infinite time either, so a few other factors come into play.
Another involved intensive material property is thermal conductivity. Thermally conductive materials have a higher heat capacity (takes more energy to heat them up because of transmittable range) and also increases the effect of other factors, mostly emissivity and thermal bridging. These can setup a very big total heat capacity over time as heat is conducted from the source and radiated away into a large environment.
So the plastic is a great thermal insulator, and with the thin walls it doesn't take a lot of the heat to warm it up. With a lid the temperature drop should be a nice shallow slope.
The ceramic is a thermal insulator, but not as good as the plastic. It does have thicker walls, which certainly help the insulation value, but will still take more energy up front. A ceramic dripper pre-heated should compare OK with the plastic for short intervals of time.
A stainless steel version would have low heat capacity, but high thermal conductivity. Preheats don't do as much for the metal and plastic because their overall heat capacity is low, and the metal dripper will radiate it away quickly.