There was a slight flaw in my Full Adder where one of the output bits was being set when it shouldn’t.
The cause, a slightly lower voltage caused by the diode in my OR Gate. Without the same voltage at the base of the PNP as in the emitter, one of the PNP will always remain active.
Because of this, I’m going to have to rework my OR gate as well, or the signal ‘booster’ circuit I added to my XOR gate to guarantee that it remains the same for both inputs.
Slightly modifying the AND gate, removing a resistor and replacing the right NPN with a PNP, I get a high signal whenever both inputs have enough voltage to trigger the NPN transistors.
The OR gate is similar to the AND gate, only either transistor should short circuit the base of the PNP to ground.