A DESCRIPTION OF THE PROBLEM :
Chapter 13.1 in the JLS mentions:
11. A construct emitted by a Java compiler must be marked as synthetic if it does not correspond to a construct declared explicitly or implicitly in source code, unless the emitted construct is a class initialization method 
and
12. A construct emitted by a Java compiler must be marked as mandated if it corresponds to a formal parameter declared implicitly in source code

For parameters, this information is stored in the MethodParameters attribute. However, this attribute is only emitted when compiling with the -parameters flag or if the method is a canonical record constructor.

This behavior can be observed by either looking at the produced class files using javap, or by using the Parameter#isImplicit()/isSynthetic() methods.

Additional context: https://mail.openjdk.org/pipermail/compiler-dev/2022-May/019783.html and https://mail.openjdk.org/pipermail/compiler-dev/2022-June/019924.html

I'm able to provide a fix for this.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the code below with javac without -parameters flag.
Run the code with java A.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
----------
Executable: A$Inner
Parameter at position 0
parameter.getType() = class A
parameter.isImplicit() = true
----------
----------
Executable: A$1
Parameter at position 0
parameter.getType() = class A
parameter.isImplicit() = true
----------
----------
Executable: valueOf
Parameter at position 0
parameter.getType() = class java.lang.String
parameter.isImplicit() = true
----------
----------
Executable: A$R
Parameter at position 0
parameter.getType() = int
parameter.isImplicit() = true
Parameter at position 1
parameter.getType() = float
parameter.isImplicit() = true
----------
----------
Executable: A$E
Parameter at position 0
parameter.getType() = class java.lang.String
parameter.isSynthetic() = true
Parameter at position 1
parameter.getType() = int
parameter.isSynthetic() = true
----------
ACTUAL -
----------
Executable: A$Inner
Parameter at position 0
parameter.getType() = class A
parameter.isImplicit() = false
----------
----------
Executable: A$1
Parameter at position 0
parameter.getType() = class A
parameter.isImplicit() = false
----------
----------
Executable: valueOf
Parameter at position 0
parameter.getType() = class java.lang.String
parameter.isImplicit() = false
----------
----------
Executable: A$R
Parameter at position 0
parameter.getType() = int
parameter.isImplicit() = false
Parameter at position 1
parameter.getType() = float
parameter.isImplicit() = false
----------
----------
Executable: A$E
Parameter at position 0
parameter.getType() = class java.lang.String
parameter.isSynthetic() = false
Parameter at position 1
parameter.getType() = int
parameter.isSynthetic() = false
----------

---------- BEGIN SOURCE ----------
import java.lang.reflect.Executable;
import java.lang.reflect.Parameter;

public class A {
  public static void main(String[] args) throws Exception {
    // implicit
    print(Inner.class.getDeclaredConstructors()[0], true);
    print(A.class.getDeclaredField("anon").get(new A()).getClass().getDeclaredConstructors()[0], true);
    print(E.class.getDeclaredMethod("valueOf", String.class), true);
    print(R.class.getDeclaredConstructors()[0], true);
    // synthetic
    print(E.class.getDeclaredConstructors()[0], false);
  }
  static void print(Executable executable, boolean implicit) {
    System.out.println("----------");
    System.out.println("Executable: " + executable.getName());
    Parameter[] parameters = executable.getParameters();
    for (int i = 0; i < parameters.length; i++) {
      Parameter parameter = parameters[i];
      System.out.println("Parameter at position " + i);
      System.out.println("parameter.getType() = " + parameter.getType());
      if (implicit) {
        System.out.println("parameter.isImplicit() = " + parameter.isImplicit());
      } else {
        System.out.println("parameter.isSynthetic() = " + parameter.isSynthetic());
      }
    }
    System.out.println("----------");
  }
  class Inner {
    Inner() {}
  }
  Inner anon = new Inner() {};
  enum E {}
  record R(int a, float b) {
    public R {}
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use the -parameters flag for compilation (does not work for the record ctor).

FREQUENCY : always