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I found a workaround. (Clearing the cache didnt work for me.)
But what has worked for me is when the "Pin search Results" Stops working, I go to View>Tabs Layout, then I change the layout to "Vertical Tabs" and then click "New Docked Window"
I close the New Docked Window that appears. Change the Tabs Layout back to "Horizontal Tabs", and then try "Pin Search Results" again, and it works. ((Might not need to change the "Tabs Layout." I suspect the "New Docked Window" might be all that is needed to get it working again but I didnt test that.
For me, it seems to stay fixed for several months and then somehow stops working.
Anyway, I hope it helps.