Increase armor Editor

Hi, I can’t increase the armor with triggers in the editor, no matter which value I insert or operation I select (set, add, multiply, divide) the total armor doesn’t change.

Example: I tried to increase the pierce armor of a skirmisher by 4 points.
Now instead of 0/3 it has 0/7-4 which is the same of 0/3. I don’t get it, what I have to do to put the armor to 0/7?

Here’s the issue (it’s a vietnamese skirm that’s why it has 36HP)

Thank you in advance