I'm Stuck

Here's one for all of you that I just can't figure out.  Symptoms (see Photo 1 below) are on fruiting primocane blackberry, under tunnels.  Maybe 5% of leaves affected, mostly limited to the bottom third of the hedgerow but issue does occur in bunches.  Temperature-wise, as you all know, we've had warm weather the week prior to this one.

I fully expected this to be eriophyid mites, who tend to gather around leaf veins and leave this distinctive stippling, but surveying these leaves under the dissecting scope, even all the waaaaay up to the highest power, revealed nothing, no disease signs either by the way.

Interesting too that the obverse side of the leaf shows no signs of anything (Photo 2).

We've got a pretty elite group for readership here, so please send me your thoughts if you have any ideas.  Because yes I am stuck.

Odd stippling on tops of leaves of primocane blackberry.

Obverse of same leaf is not at all affected.

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