Thanks, Kent and Undiscovered!
The ‘Clear Image’ checkbox indeed solved this issue.
I, like Undiscovered, also had difficulties using the same COLOR, CAVITY, and NORMAL maps for both metallic and wood materials. The way I approached this was through vertex colors (Kent, you hinted at that in your week 4 live stream): I gave the wood materials a black vertex color and metals a white vertex color. Then I used the the vertex color input node in the material node editor to apply separate materials.
