Until flag not working with Until activity of ADF and loop keep going

If you look into the official MS docs on UntilActivity

Enter an expression that will be evaluated after all child activities defined in the Until activity are executed. If the expression evaluates to false, the Until activity will execute all its child activities again. When it evaluates to true, the Until activity will complete. The expression can be a literal string expression, or any combination of dynamic expressions, functions, system variables, or outputs from other activities.

So try with this expression to evaluate condition

@equals(variables('until_flag'), 'false')