Overflow bottle melting

Is your gauge showing CHT or does it have a sensor in the coolant hose somewhere? If it is the latter then it will under-read if you lose the coolant. 

I have seen a melted expansion tank once when the coolant radiator was covered so there was insufficient cooling and the coolant boiled and overflowed, melting the bottle in the process. The separate CHT gauge did show temperatures in the red during this episode. 

Perhaps you have a blockage in the coolant system or the pump isn’t working. 

It’s a 1996 912UL with the old CHT rather than the new measuring of actual coolant. The engine has 350hours in last 3 years and has been actively flown in that time.  It did a 6 hours non stop flight the day before with no problem in quite hot outside temps.

I was carrying out and extended climb and then looking at full power on stall characteristics while also attempting to get oil temp above 100°c as recommended to boil off condensation. during this exercise which took some time, the rpm was 4800 at full power and speed was 40-50 kts.
The aircraft may not have been flown like this before however.

Perhaps more cool airflow around the reservoir would prevent this. Hot coolant alone should not melt it.

I have seen this, but not from the coolant temp, but from exhaust leaks and too close to exhaust pipes.

When I was a kid, we used to play a game where we would fill a paper cup with water and place it right in the flames of a campfire. The paper cup would not be damaged until the water boiled off. The liquid absorbs the heat. Only when the liquid is absent can the paper cup be destroyed by the fire.

I do not believe it is possible for hot coolant to melt the container.