Let's consider following code:

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.Arrays;

interface MyFunctionalInterface {
    Object invokeMethodReference(Object... args) throws Throwable;
}

public class Test {
    public Object m(Object ... args) {
        System.out.println("m: " + Arrays.toString(args));
        return "m";
    }
    public static void main(String argv[]) throws Throwable {
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        MethodType mt = MethodType.methodType(String.class, char.class, char.class);
        MethodHandle ms = lookup.findVirtual(String.class, "replace", mt);

        System.out.println("result: " + ms.invoke("some string to search", 's', 'o'));

        MyFunctionalInterface methodRefSimple = new Test()::m;

        methodRefSimple.invokeMethodReference("first", "second");
        MyFunctionalInterface methodRefSignPoly = ms::invoke;
        System.out.println(methodRefSignPoly.invokeMethodReference("some string to search", 's', 'o'));
    }
}

it causes following output:

result: oome otring to oearch
m: [first, second]
Exception in thread "main" java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle(String,char,char)String to (Object[])Object
at java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:776)
at java.lang.invoke.MethodHandle.asType(MethodHandle.java:770)
at java.lang.invoke.Invokers.checkGenericType(Invokers.java:366)
at Test$$Lambda$2/1761291320.invokeMethodReference(Unknown Source)
at Test.main(Test.java:26)

The following line:

"System.out.println(methodRefSignPoly.invokeMethodReference("some string to search", 's', 'o'));"

causes java.lang.invoke.WrongMethodTypeException to be thrown.

JLS doesn't prohibit method references to signature polymorphic methods in any way. So I could believe it's a bug.

The code above is attached for your convenience.