Hi
I've tested this and can confirm a right square bracket "]" will be lost from the input fields in a trigger copy and paste procedure as you describe. I will raise a support ticket for this.
I would also mention rubold is correct with regard to not using square brackets in the set user variable trigger action, if you use square brackets for setting user variables they may not parse correctly. Please see the attached screenshot where:
user variable mv1 is set as value [market_volume]
user variable mv2 is set as value market_volume
and the resulting formula
you are quite right though for using square brackets for variables in the text of the trigger actions write to file, email, text etc.