That's interesting! I'm curious to know: is there a bend in your wrist while typing (either with or without the wrist rest)? Also, are your arms parallel to the floor? Also also, do you rest your elbows on anything?
In case anyone else comes into this comment section looking for ideas: the thing that helped me the most was adding soft pads to my chair armrests - I found that resting my elbows on the hard surface of the default armrests resulted in forearm pain.