Forearm moving behind Upper Arm

If you draw the upper and lower arm in one layer it is a bit random. What you need to do is group the artwork into layers. It does not need to be independent, just in different layers/groups so CH knows what should be in front of what. Eg split the arm into two groups at the elbow, the make sure the forearm group is above the upper arm in the layers. Ch always shows the higher layer in front.

. Arm

. . Forearm artwork

. . Upper arm artwork