If I'm understanding right you mean the bottom of the tub has a bit of a curve upwards right? If so that's how mine is too when I bought it and part of the design.
Was the swell present before the heatpad was set up? Keep in mind when you sandwich the thermostat probe between the bottom of the tub and heatpad there will be abit of a "arch" or "air pocket. If this is the case I don't think you need to worry.
If anything you may just need to put sheets of styrofoam underneath it to help have the heat pad flush against the bottom. As long as the heatpad is regulated by your thermostat the plastic won't melt. If you think about it human body temperature is around 96. If the plastic was so sensitive to heat it would start melting in our hands aha.
All plastic are different but generally melting temperatures are around 200F and above minimum. Not that you wanna get anywhere close to that but it gives you an idea of what temperature are dangerous as far melting goes.
Edit: Also if you don't have one already I recommend getting a temperature gun to measure surface temperatures accurately.
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