Just a guess, but your burnout cycle is probably both too hot and too fast. If you can get an MSDS for the specific resin, it will provide melting and oxidation temperatures of the particular resin / plastic.
You want to get the material to melt and flow down out thru the button hole. It might take several hours. You also want to do it slowly so that the chemistry of the resin doesn’t expand to much and also compromise the investment. The coefficient of expansion is higher for plastic materials than for wax. It’s also acidic on the investment.
There’s no exact science, but I would slow down your burnout cycle. Once you slowly it to 200-225 or whatever the MSDS indicates, let it cook for 4-6 hours. Then do several more hours for the regular ramp and hold cycle. I’ve noticed fumes will after that 5 hour point a couple of times, so slow you go. You’re probably looking more at an 9-12 hour burnout to end up at 1200F with a couple hors holds at the top temperature. Big temperature swings / fast ramps should be avoided.
I’ve played with ultra thin styrene sheet with some success. It’s one reason I invested in my own studio, to precisely control the burnout cycle which I discovered was crucial to a design’s success.
And you might want to use an investment engineered for platinum. It’s alot stronger in the burnout cycle for more resinous plastics.
And because the fumes are nasty, a ventilation exhaust to get fumes controlled and properly vented.
Did you try casting the flaming flask? It might be ok if you redo the burnout at a much slower ramp and hold. I’ve had flasks go freaky on me a couple of times and I let them cool overnight and redid the burnout with success. Was your job damaged?