I am not a pro sign cutter. I do have a fair understanding of polymers. Polyethylene is a thermoplastic and very susceptible to temp above 250 f and deflection above 150. When machining temp at the end of the cutting bit can gum up and heat up. This will cause all kinds of havoc, color, tecture,detail etc. Because you are following manufacture specifications for this material and bit.I have to wonder if you have a dull tool. I know Bill spoke to tram and said it’s not an issue but it would not hurt to check .
Dave