After Elsweyr , ALL SOURCES of major protection are causing Health Desync no matter if you are fighting 1v1 or 1v5, your Health will stop dropping and eventually you will die on 90% health (what u see),...
I didn't experience more health desyncs while wearing Pirate Skeleton than without said set (no other source of Major Protection). So i don't think it is an issue with all sources of Major Protection and 100% of the time (not saying there can't be an issue at all tho).
What's bugged also with pirate skeleton is the animation, each time i change into skeleton i'm stuck from swapping bar for like 1 sec or more. Stopped using it because of that.