Just got the Diggle Nog achievement, so evidently it's not a case of achievements in general not working.
Yeah, I've literally tested every skill tree with a savefile that had a few points to burn. Not a goddamn one will trigger the achievement when maxed. The DLC specific achievements just seem to be broken all across the board. Diggle Nog was a pre-DLC achievement and I achieved it prior to downloading the DLC.