Percolation? Heat soak? Don't drive during summer??

my alternator is definitely charging

In common lingo, people often say 'alternator charging' when they mean alternator is supplying power to the car.
That's NOT what you are seeing on the "alternator" gage here.
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This is showing the alternator supplying power to the battery.
D = Battery Discharging
C = Battery Charging.

Notice where the ammeter is in the circuit?
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Its between the battery and the main splice.
Current flows through the ammeter only when the battery is discharging or charging.
With a fully recharged battery and a working alternator, no current flows through the ammeter.

More charging with rpm means one of two things.
1. Battery is extremely low on charge or is damaged.
2. Voltage isn't properly regulated so its going up with rpm.
Fix this because it will cause all sorts of problems, some of them potentially disasterous.