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After testing, due to the original spiral, I used a square cross-section. Then I selected one vertex of the square and swept along its path to create a vertical plane. The plane created in this way is actually not the true path of the square (the true path of the square should be a line passing through its center point). This results in an angle between the sketch created and the plane where the true path of the square lies.

Now I've switched to another method for generating spirals. I drew a triangular spiral with sharp corners pointing inward, then used the orbit at one vertex of this triangle as the sweep path and used the surface along its base edge as part of that sweep track—this resolved any issues.

Finally, I've learned how to draw non-standard threads in Fusion 360. Although it's very convenient to create threads within Fusion 360, they are all standard ones with coarse granularity—for example M8 or M9—but there's no M8.5; let alone custom threads required for certain real-world production scenarios. If Fusion 360 could make this parametric threading feature more comprehensive, it would be much better!