I have 2 level 50 glads with surprise ending

One w/ Surprise Ending Lvl 10
The other level 8

The thing is - they NEVER use surprise ending. I mean - in like a whole week of fighting - not once.

Does it even work? Is it broken?

I even changed strats from counter - to Parry - to -slash - to lung - and nothing.

Both defensive glads.