issue with setting ?cancelable - SOLVED, but why?

I edited the line:

fail_trigger_result := SetFailTrigger(timer)
if(fail_trigger_result = true):

into

if(SetFailTrigger(timer)?):

And my code started working.

I then hit undo and retried, and the code is still working.
I don’t understand.

-edit
It’s gone back to not working now.