Offense up and Offense down combined = offense down?

They should cancel eachother out. Both go off of base offense as in offense up will add +50% of your characters offense before buffs/debuffs, and Offense down should give -50% of that same offense number before any status effect buffs/debuffs are taken into account.

Is it possible you're just attacking ships like Sun Fac when you've got both status effects? Sun Fac has a lot of armor and gets extra armor when attacked by buffed enemies.