Emptiness

I have no idea, but it definitely sounds like a bug.
 
Please provide the information and files requested in this thread: MUST READ: How to Report an Issue
 
Also, please indicate whether you have installed any mods at any time while playing with this party, or if you have used any method (savefile edits or console commands) to modify your characters or party, and if possible detail what changes you made.

The early design of Monks had some of the damage inflicted on a Monk converted to wounds (ie they would take less damage from hits against them).  Then, if they did not expend the wounds to fuel special attacks, eventually the wound would be converted to damage to the Monk.
 
That design was changed to the current system.  Monks take full damage from attacks against them, but earn wounds based on the amount of damage they have taken.  They can use or not use those wounds as they see fit.  The wounds mechanic is not an inherent source of damage mitigation.