Perhaps it’s by design, but (if I recall correctly), the Exalted has a max of 25 Will Power. However, during the final mission she was hiding behind the 1st and 2nd walls along with a priest with the Synod Head (allied units within 7 tiles recover 2 Will Points per turn) while the rest of the team mopped up the baddies. By the time we moved into the final staging area, the Exalted had a total of 53 Will Power.
EDIT: This may be by design. Exalted has a max will power cap of 28. I tested and she can accumulate an extra 28 for a total of 56. Then again it might still be bug.