Odd colour banding on printed gradient

Our signage company produces vehicle graphics. Last week we printed graphics for a van and got some unexpected results.

The print file was a pdf, and had a vector of a snowflake, as a compound path. Under the vector of the snowflake was a long drop shadow (as a vector shape, not an effect) with a CMYK gradient fill. The snowflake shape had a Pantone Spot colour fill applied, and was set to "Overlay" for the blending mode.

When viewed on screen, the underlying gradient, which was visible through the overlayed blend mode, looked fine. We did a test print, and that was also fine.

When we printed the final artwork, the was a lighter band of colour toward the darker in of the gradient.

(see attached images)

We tried several different fixes, including flattening the transparency, which also produced mixed results. Sometimes the band was eliminated, but then the overall colour did not match the original print.

Our solution at the time was to print it as an AI instead of PDF, but we're unsure if this will fix the issue, or if we just got lucky.

We've experienced this problem a few times now and we're wondering if this is a known issue; and if there is a solution.

We are currently on Illustrator 26.0.3

and are printing with Caldera "GrandRIP+" OEM Durst 10.2

to a HP Latex 560.

I have attached photos, the print file and the PDF Preset we used for the print.

Edit: We also got the same result on another job today. This time with no overlapping shape, with blending mode. Just a flat CMYK gradient. Interestingly both files have blue gradients (also see attached files)