Interaction between @PolyNull and method type argument inference

Take:

import org.checkerframework.checker.nullness.qual.PolyNull;
import java.util.function.Function;

interface Transform<S, T> {
  @PolyNull T transform(@PolyNull S in);
}

abstract class Demo {
  abstract <A, B> Function<A, B> map(Function<? super A, ? extends B> f);

  <C> C then(C in) { return in; }

  void use(Transform<Object, String> tf) {
      then(map(x -> tf.transform(x)));
      then(map(tf::transform));
  }
}

The signature of Transform::transform ensures that the nullness of the parameter and return type are equal, ignoring the nullness on the type arguments.
Functions map and then have signatures similar to many streaming APIs.

Running the Nullness Checker gives:

PolyTest.java:15: error: [methodref.return.invalid] Incompatible return type
      then(map(tf::transform));
               ^
  Method
    @Initialized @Nullable String transform(@Initialized @NonNull Transform<@Initialized @NonNull Object, @Initialized @NonNull String> this, @Initialized @Nullable Object p0) in Transform
  is not a valid method reference for
    @Initialized @NonNull String apply(@Initialized @NonNull Function<? extends @Initialized @Nullable Object, @Initialized @NonNull String> this, ? extends @Initialized @Nullable Object p0) in java.util.function.Function
  found   : @Initialized @Nullable String
  required: @Initialized @NonNull String
1 error

Note that the error is only raised for the method reference.
The equivalent use of a lambda doesn't raise this error.

This is probably related to #979, but I didn't see any tests that use @PolyNull in checker/tests/nullness/java8inference.