Overflow bottle melting

The bottle that I saw melted wasn't a genuine Rotax one but was one made of a softer type of plastic that obviously couldn't withstand 100+ degree water.

I’m sorry but did you say full power (Wide Open Throttle?) in your aircraft is 4,800 rpm? 

It is suspicious that the only melting was at one corner and not the whole bottom of the bottle.
Without the pressure of the coolant system, 50/50 Antifreeze will boil and not get any hotter than ~225°F in the bottle.
I would suspect the melting was caused by an external heat source and happened AFTER the bottle was already empty.
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WOT = 4800 rpm and 40-50 kts. ???
What aircraft is this?

No. Full power WOT straight and level is 5600rpm

In a 30° full power climb at stall it was 4800

Zodiac 601HDS