Google can't please all of the sites all of the time, or all the visitors.

It is too big to evem worry about that 4k a day clicks for one site. It is like us optimizing the expense of 0.01c. It makes a difference when that 0.01c is an API call that you call a million times. But it only surfaces if you do aggregate it.

Therefore this problem can only even be seem by Google if it can be surfaced in aggregate overy say a billion queries.

I wonder how that can be done.

Probably only can be done using data. Which means spying on people in various ways. And making assumptions about length of time on site equals quality.

They probably use machine learning too. There may be no reason for the lost rankings other than a wind change caused by some updated parameters in an OKR chasing model.