We have had our Fuse1+ for two years now, and the Fuse1 for three years.
Did you manage to get this sorted? If yes, please let us know your solution so it can help others aswell. Birch bark can be caused by several issues but its always good to know all the tricks to solve it. 
Due to the 15% more print armor, the print quality has significantly improved. We have now also conducted a test under nitrogen with the 15% more print armor and are thrilled with the result. The surface is almost as good as on the first day.