looks like the nailing pattern for plywood
not tyvek.
even with tyvek you have exposed nail heads.

it is possible that tyvek isn't installed
impossible to know without taking a piece of
vinyl off to see.

what is your location?

Depending on the pH of the rain it could just be zinc from the nails holding the siding itself.

Does it wash off?

You could even try some white vinegar and see if that takes it off.

the 3 things I thought of were
frost on nail heads if the condition happens in cold weather.

oxidation of nails

acid rain

post back OP and let us know where you are located.
and if you are near salt water...

We live in Milwaukee, WI. Is it worth doing an insurance claim?
They can't get rubbed off or washed off.

It does not appear to be a sudden problem, but a long term maintenance issue.

Not what insurance was designed to cover, but you might get luck and your rates not go up excessively (or your house get branded 'not insurable').

Did you try some distilled vinegar?

It dissolves zinc leaching nicely.

You've been asking this question for years. I thought the siding contractor's son was going to help you make a claim for defective siding.

Vinyl/PVC is not particularly strong or durable; it's main claim to fame is that it is inexpensive. If the plasticizers used are not of high quality, or the manufacturing process went awry, the PVC can become prematurely brittle and perhaps that process is affected by the support brackets or nails. Thin PVC can be dissolved by certain solvents used in sealants and self adhering waterproofing membranes. Also that strange texturing that makes PVC siding look like poorly surfaced wood clapboards is a good surface for fungus/mildew to grow.

I misread the date of another post on another forum so perhaps this problem is a new one.

I found your lack of patience and courtesy with people offering free advice to be troubling.

I haven't been asking this question for years. Lol. Wow