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Can you confirm that you have Significant Locations enabled? Settings > Privacy > Location Services > System Services > Significant Locations.
 
I recently cleared significant locations on my 12 Pro Max in order to aid optimized charging and it has yet to log any places. Anyone have advice ? Yes all settings are on !
 
In my case my list of significant locations remains empty even after multiple visits of long duration to many locations.
 
After struggling to get Optimized Charging working on my 11 Pro, I made certain when I got the 12 Pro to do all the things I knew needed to be done from day one. It took two weeks (exactly) before Optimized Charging started working on the 12 Pro.
 
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