That is strange, since it does not work in any battle.
I made some screenshots to show that it does not work in another battle either.
https://www.dropbox.com/s/jzlhzkuzqxeeze4/POE_Withdraw_screenshots.rar?dl=0
Here Durance has withdraw cast on him, he currently has 125 endurance, and the second screenshot is three seconds later, and the third is eight seconds later and his endurance is 125 in all of them.