The base is made out of ptex and yes, you can absolutely melt the base if you apply too much heat.
Sounds to me like that shop fucked up your board and they owe you a new one. They're trying to pass off the blame and play dumb, but they know exactly what happened.
Someone left the gun laying on the board or some other form of carelessness that they will surely never admit to you.
Get it replaced!
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