Also, the reason for capping Martyr wraps and not corrosion is just for simplicity; applying random rules to status effects gets very messy, very quickly.
Well, considering burning is already capped in such a fashion I hardly see a similar cap on corrosion as out of place.
Remember that corrosion effects the player with no limits, why would there be limits to monsters?
Rules work differently between players and monsters for a lot of reason. Primarily, the player has the benefit of initiative and decision and can choose to avoid status ailments if desired. Monsters have no such luxury. This is also why poison is only half-effective against fast-regen monsters but totally effective against monks, and why players have a resistant cap but monsters to do not, and why monsters always win on mutual first strike or equal level engagements. The player is the one that chooses when and how everything happens, so monsters get preferable rules. How'd you like it a level 9 monster decided to harvest you
Secondly, players can remove a stack of corrosion with burn salves. There's no equivalent "curing" process for monsters.