Snowbird writes:

Do you have a reference saying this is always the case?

Every reference I've checked says so. They sink at a variable rate, but
usually at least 150-200 feet per minute, sometimes much more.

They have to do this, because it is the reaction to forcing the downwash
downward that produces lift.

Maybe the propwash?

I'd expect the propwash to be drawn down with the downwash, but I'm not sure.
I would not expect the propwash to be significant after two minutes.

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