What you’re running into here is that once a release is created, those variable values are set for that release and any changes you make to the variable won’t be reflected until the next time you create a release and deploy it.
I think in your case where you are trying to access the variable in a later step, you will want to use Output Variables. These will allow you to set a variable in one step, and retrieve that value in a later step of the same deployment.
Please let me know if that helps or if you have any questions.