Noticed this several times last night: the original SvS ship invincibility glitch is back. Enemy ships can be "hit" with damage scored against them, but their health meter never budges. They remain in the game, are still able to sail around, still able to have people board, etc.
Pictured here is after about 30 volleys hitting the aft: 90 shots doing 360 points of damage each. (You can see at least one "-360" over the poop deck. Sail points presumably were damaged before entering the glitch; hull points remain full.)
The first ship I saw doing this glitch last night, was a captain who avoids glitching (from my conversations with her, at least) and she was unable to get out of the glitch without logging out (taking many thousands of gold doubloons with her when she left, contrary to her normal habits.)
While this has occurred irregularly in the past, after an hour or so last night, a half dozen ships mysteriously all had the same glitch.