You are hitting https://github.com/JuliaLang/julia/issues/14821. Change
c = coefficients[1] * x1_1 * x5_1 * x6_1 * x7_1 * x10_1 * x11_1 * x12_1 * x15_2
to
c = coefficients[1] * (x1_1 * x5_1 * x6_1 * x7_1) * (x10_1 * x11_1 * x12_1 * x15_2)
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