Atmega32u4 Pin F0 output is only 3.64V, not 5.00V
Suggestion 1: Check your ground connections.

Suggestion 2: Edit your question and show the external circuitry, if any, that's connected between F0 and GROUND, including a fixed resistance between F0 and GROUND that models your voltmeter's input impedance.

Suggestion 3:  Measure / determine the DC output current at pin F0. That output current must not exceed 40 mA. Also, a DIO pin's output voltage is inversely related to its output current: the greater the output current, the lower the output voltage.

Suggestion 4:  Verify you are not exceeding the chip's max rated DC current spec of 200 mA at the VCC power pin--e.g., the combined sum of the output currents at all DIO pins does not exceed, say, 160 mA (see also the chip's ICC electrical spec).

Suggestion 5: Your post doesn't state whether output pin F0 is programmed for logic HIGH output, logic LOW output, or is tri-stated (floating).