Voltage divider and floating voltage read

As you are using a UNO the default reference is Vcc.
So the result from the ADC should be 1023 for Vin = Vcc.
Even if Vcc is pulled down by a heavy load the ADC reading should still be 1023.
The only way you would see what you are describing is if the connection from the 5V pin (red wires) to the top resistor & ADC had a resistance comparable to R1+R2.

I'm uncertain how you expect to do that. Perhaps you could post a schematic?

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