pre-required state always runs twice regardless whether changes empty or not

According to the documentation

If the "changes" key contains a populated dictionary, it means that the pre-required state expects changes to occur when the state is actually executed, as opposed to the test-run. The pre-requiring state will now actually run. If the pre-requiring state executes successfully, the pre-required state will then execute. If the pre-requiring state fails, the pre-required state will not execute.

If the "changes" key contains an empty dictionary, this means that changes are not expected by the pre-required state. Neither the pre-required state nor the pre-requiring state will run.

The fact is that the pre-required state always runs whether there's any change or not.
The pre-requiring state only runs if changes are expected.

Lets make some clarification with the confusing terms used here:

test-a:
  salt.function:
    - name: service.stop
    - tgt: minion1
    - arg:
      - myservice
    - prereq:
      - test-b

test-b:
  salt.state:
    - tgt: minion1
    - sls:
      - mystate

In this example, test-a is the pre-requiring state, test-b is the pre-required state.
What happens it that test-b always run twice whether changes are expected or not.
Under normal situations, this doesn't hurt anything, but it would cause serious problem if test-a fail to generate changes in test mode, as in #49753 .

I know that the pchanges issue has already been fixed, but it would be nice to make prereq work as expected, or at least update the documentation.

Use the test-a/test-b example to trace prereq execution and compare the documented populated-versus-empty changes behavior with the actual runs. Review the related pchanges fix and issue #49753 for context. Done means prereq states follow the documented execution rules, with coverage for both changes cases, or the documentation accurately describes the retained behavior.