Pin#18 VT/OR color, +12v with LO beam
Pin#20 RD/OR color, +12v with HI beam and FlashToPass
With headlamps on, while toggling between HI/LO beams, the +12v should alternate between pins 18 and 20.
Pin#18 3.75v reading may be the BCM input which monitors the low beams.
Pin#20 5.35v reading may be the Daytime Running Lamp Module input which monitors the high beams.
Your readings sound normal.
Did you measure at the headlamps?
Headlamp HI/LO beam circuits are each controlled (spliced) together. One of your headlamp plugs/wiring might be miswired.
Check the wires at the headlamps, test for +12v with meter:
HI beam = +12v on RD/OR wire
LO beam = +12v on VT/OR wire (center pin)