For some reason, I was unable to open your replay.
Just to be sure, you used the open to disengage at least one friendly unit correct? There is a "and then" clause which states that you are required to meet the first clause to activate the second part of the ability.
It never allowed me to select a unit to disengage them, and it never selected one itself. it has worked when i played it before. thanks for helping though.
There is no selection process, the system checks whether you have a unit that's engaged or not and disengages if it found any. If you do not disengage any engaged friendly on your side of the field, you will not engage the enemy with this skill.
So, did you have an engaged friendly unit on your side of the field or not when this skill activated?
EX Maggie (in the Gowen pack) has an ability that will engage your units for 2 turns if your opoonent uses it. It's not common, but some units do have these abilities.