After re-reading the tool-tip for Buccaneers a few times, I think I was mis-interpreting how this unit works. Originally I thought it meant that when attacking a unit behind walls, the defending unit gets no fortification bonus. After posting that I noticed when a second Buccaneer attacked the same unit the bonus was gone, I am now interpreting (hopefully correctly this time) that the fortification bonus is removed after the Buccaneer has it has attacked. Hopefully this explanation will help anyone else that had the same reading comprehension problems I did.
Attacking an AI unit behind city walls with a Buccaneer. When looking at "Attack Prediction", +6 Fortification still applied to defender. This was with the first Buccaneer that attacked in a siege.
Interestingly, a second Buccaneer attacking the same defending unit (after having been attacked by the first Buccaneer) did remove the +6 Fortification.
Also, attempting to attach Bug Report zip file, but getting an error that file is too big (larger than 10mb), even though file is just under 8mb. I even tried zipping just a few of the files and re-uploading to get it under 4mb, but site is complaining file is still too big. Seems bug reporting is bugged too.