Note: I have come up with a solution, but it feels dirty. So..
I have this controller file

class SignupController extends GetxController {
  static SignupController get instance => Get.find();

  final checkbox = true.obs; 
  
  // this function returns the value correctly, either true or false
  testOne (){
    print(checkbox.value); // this value changes
  }

  // this function always returns 'TRUE' regardless of the changes
  Future<void> signup() async{
    try{
       if(!checkbox.value) {
        mySnackBar(
          title: 'Accept',
          message: 'To create an account please accept'
          );
          print(checkbox.value); // This value is always 'true'...why?
          return;
      }
}

I call the function like this

onPressed: () => SignupController().signup();

// I change value like this
Checkbox(
    value: privacyPolicy.value, 
    onChanged: (value) {
    controller.privacyPolicy.value = !controller.privacyPolicy.value;
    }
)

NOTE: the observable state changes on the screen. check box tick works fine, its when I try to get the current value that I get this problem

My workaround was to bypass the controller class and pass the value of the checkbox manually… i.e.

Future<void> signup(checkbox) async{
    try{
       if(!checkbox.value) {
        mySnackBar(
          title: 'Accept',
          message: 'To create an account please accept'
          );
          print(checkbox.value); // This value now returns correctly
          return;
      }

//// and defice checkbox here
Widget build(BuildContext context) {
    final controller = Get.put(SignupController());
    Rx<bool> checkbox = true.obs;

But I want to have my controllers in my controller file