Heavy oil plus an undersized wick is a common tunnelling cause
Heavy resinoid oils above three hundred centistokes thicken the melt pool and slow the capillary draw of the wick. When the chosen wick is already at the top of its tested diameter range, it has no headroom left to compensate, and the melt pool fails to reach the wall on the first burn. The rule fires when blend mean viscosity is above three hundred centistokes and the vessel diameter sits within five millimetres of the wick's maximum tested diameter. Step the wick up one size or move to a paper-cored family.
What this means for your recipe
When this rule fires the recipe loses 15 points and drops a band. Worth addressing before selling. The same rule fires every time the underlying condition is true, so the way to lift the score is to change the recipe, not the rule.
Primary sources
Editorial content, not legal advice. Verify against the current text of the regulation before commercial sale.